PHILADELPHIA - The right leg-kick is slow and high, the knee bent to the level of his chin. His slender upper body rotates left and rear. At the apex of his motion, he harbors his power - a kind found less in muscle-bound strength than in movement reverently mastered.

Natural as a waterfall, the leg descends, the torso follows, the front foot anchors in the earth. A ball erupts from his left hand, sizzling. How hard it travels, nobody knows: No radar gun can track it.

This Philly ballyard is thick and green, lush with summer on a steamy late Sunday afternoon. The pitch zips across it - so flat, so level, so low it seems to riffle the grass. No batter - not Tony Gwynn, not Tony Batista-would take a cut. It's too far down. The hitter slumps, relaxed.

But to the skilled in this growing game - like the Stompers of Gaithersburg, clad today in their trademark Dodger blue - a play is never over till the last millisecond. A foot before it reaches home, the ball jerks up like a frightened horse. It leaps, all but vertical, through the strike zone. Bang! Strike three, inning over.

It's early in the game, but Danny Isenberg, 20, the Stompers' ace southpaw, has his seventh strikeout. He trots to the bench, head lowered. It's the title match of the all-day tournament.

Void of expression, he sinks into a lawn chair. It's in him to fool a hitter. It isn't in him to show one up.

If the ball he threw were cork, horsehide and stitching, "Dan-o," as teammates call him, would be worth millions. His whippet frame, graceful motion and smooth delivery call to mind Ron Guidry, the former Yankee Cy Young winner. His mastery of four pitches, each thrown from multiple arm angles, makes him harder to hit than a big-league star.

That's partly due to the physics of his game: wiffleball.

At five and a quarter ounces, a baseball is a whirligig in the hands of a Guidry or a Glavine, but at least it seems to follow the laws of science. A wiffleball? This 18 grams of hollow plastic, perforated with slots along one side, can be made, with zealous practice, to dip and rise, to dance and sing, to drive the greatest hitters out of their minds.

You know wiffleballs. Like Slinkies, Silly Putty and BB guns, they're a classic American toy. Maybe, as a kid, you flung one around the back yard. You and your buddies made up rules - a folded glove was home plate, a ball off the fence a triple - and you tried out your curve, your slider and every other pitch you couldn't throw with the big-league sphere.

Some hopped like a toad on asphalt, some floated, some fell. Some splashed in the neighbor's pool. But you knew it was a toy - evanescent as boyhood, transient as a summer afternoon.

Don't remember? Just pick one up. There's nothing to it. Never has been - not since 1954, when the Wiffle Ball Co. of Shelton, Conn., rolled out the first model. Throwing a wiffleball, says Nick Schaefer, the Stompers' right-handed starter, is like throwing air.

Wiffleball - it's one of those childish things you put away. Or is it?

What's so funny?

Tim Cooke, 20, the blond, buzz-cut captain of the Gaithersburg Stompers, knows just why you ask. If the plastic ball and bat didn't bring out the kid in him, he'd never have picked them up in the first place. Sometimes pals will ask him what he's up to this weekend. "Playing wiffle," he'll say. It always draws a laugh.

That's when Cooke, a 215-pound ex-baseball star, gets his team-captain look. "I just hand them a bat and say, 'Here, take a few cuts against Danny.' When those balls start diving in from all over the place, they don't think it's so funny anymore.

"That's when they hand me the bat and say, 'Have fun.' "

Cooke and his teammates always do. Fast-pitch, semi-pro wiffleball, now one of the fastest-growing sports in the nation, has its roots in a whimsy that gives birth to teams called Wiffle du Fromage (Costa Mesa, Calif.), the Toadkiller Dogs (Dundein, Fla.), the Savage Geckos (Annapolis) and Naked Lunch (Salem, Mass.).

One of the deepest joys, though, is that "wiffle" repays those who take it to heart. Schaefer, for example, who weighs all of 132 pounds, throws a drop ball feared throughout the East. He's honed it for a decade. "I aim it at the top of the backstop," he says, "and throw it as hard as I can." From a height of 15 feet, it dives through the strike zone like a bird of prey.

"Not to sound cocky," says Schaefer, 22, "but that pitch, on a good day, is unhittable." You've seen a few today. He's right.

He'll never tell you this, but Cooke, a public relations major at Salisbury University, is one of the bulldozers behind the national game. "Wiffle" has exploded so dramatically over the past 10 years it's hard even for insiders to count the leagues, teams and tournaments at play from Walla Walla to Waco. Cooke guesses there are 2,000 serious players or more - maybe a lot more.

You might expect that, in the shadow of labor wrangling, jocks who pocket millions and rocketing ticket prices, wiffle was born as an antidote to big-league baseball. Not true exactly, but you wouldn't be far off.

People come to the game for many reasons, says Cooke, who, with his teammates, rises at 4:30 a.m. to travel to tournaments. "Mostly, it's fun," he says. "People are familiar with it. They played in the yard as kids. When they hear there are organized leagues and teams, with ballparks, standings and rules, they think, 'Hey, we can do that.' They'll form a team, which takes just three to five players, enter a tourney, and we grow a little more."

What Cooke and others like him found in the '90s was that wifflers all over the country were tossing plastic, never guessing organized play was available nearby. Bruce Chrystie, executive director of the Stompers' league, the United States Perforated Plastic Baseball Association, says when he started 20 years ago, he was just fooling around with his buddy in the yard. News spread only by word-of-mouth. He got wind of a tourney, signed up on a dare and found to his shock he was already one of the top pitchers in the East. At 38, he still anchors In The Box, a longtime New England power.

"Today, though, we're Internet-driven," he says. "That's why it's exploding. Anyone can get online, click on Google and find good wiffle within a few hours' drive." The USPPBA site, at www.wiffleball.net, even boasts an online magazine, Fast Plastic.

Hard-core wiffle can be a rude awakening for newbies, though, many of whom show up never having seen an Isenberg, a Schaefer, or a team like the Gaithersburg Stompers. But that's part of the process. "When we started," says Cooke, "we had no idea how bad we [were]. We saw pitches so hard we couldn't believe it. We had no chance.

"But it was fun. Wifflers are great people. They were encouraging. We decided to stick it out and learn. Four years later, we've arrived. We have a chance to go a long way."

Today, three of 12 teams in the Philly tourney play competitively for the first time. One, the Dodgers, consists of a dad, 48, and his two sons, 22 and 13 - a typical wiffleball range. They learned of this competition in the paper, scraped together the $100 entry fee and showed up for the dawn-to-dusk extravaganza.

They do well, winning one game, then dropping three by a run apiece. The two other newcomers - the Deer Run Dodgers and the Delaware Dillywoppers - are less successful. Their jaws drop as veterans of the circuit - the Stompers, Bronx Bombers, the New York State of Mind - cordially trounce them. "Deer Run did great," says Cooke. "I'm sure we'll see them again. Their pitcher isn't fast yet, but he throws strikes. That's the place to start."

During their 10-run win over the Dillywoppers, Cooke and Schaefer visit the other team's sideline. They show how a curve and a riser are held, how to wait longer before swinging the bat. The 'Woppers, wide-eyed, listen. Schaefer returns to his sideline. "Cool guys," he says with a smile. Across the field, the Delaware rookies are tossing it back and forth.

A little Fenway

Cooke's discovery of grown-up wiffleball was typical: It was love at first sight. A man in Bethesda had built a scaled-down Fenway Park in his yard, a towering "Green Monster" wall in left field included. Cooke saw it in the paper. "Now, there's something I can do," he thought.

He meant the game, but also the stadium. He begged $500 off his dad, drove to Home Depot for a truckful of boards and built a ballpark near his house. Before he knew it, teams were forming and wanting in. Tournaments happened. "Wiffleball is pretty easy to set up," he says.

Cooke, his younger brother and current teammate, Paul, and Isenberg grew up in the same neighborhood with clear ideas how the game should be played. There are many variations, including slow-, medium- and fast-pitch, and fields whose foul lines intersect at anywhere from 70- to 90-degree angles. "Anyone can hit a slow-pitch grapefruit," sniffs Isenberg. "The best wiffleball replicates baseball as closely as possible."

Cooke watched tourneys in Bethesda. From those games and others, he learned that pitching is a team's key asset and got ideas for a standard set of rules. Most are now in use in the USPPBA - the only league with teams coast to coast - and spelled out in the league's rule book.

Some leagues give a hitter six balls and two strikes, says Chrystie. Some give only one strike. Field dimensions differ. Some have baserunners, some don't. "Our league is the only one with uniform national rules," he says, "so when we have our Final Four, everyone knows the same game. We get a legitimate national champ."

Only one other league, New Jersey-based Wiffle Up!, has tournaments in multiple cities. Mike Alessie, the commissioner, operates 12 during the season, all of them in the East. The annual Baltimore tournament, scheduled for next Saturday in Holabird Park, will have at least 32 teams. "We have a one-strike rule," says Alessie. "It keeps things moving. We play a lot of games in a day."

The USPPBA has its own ways. First, it's all fast-pitch. The rubber is 40 feet from home, and some hurlers bring it at 75 miles per hour. (That's an estimate; radar guns don't register perforated plastic.) The foul lines are set at a 90-degree angle. Fifty feet divide the bases, which for cosmetics' sake must be official white baseball sacks. Runners are imaginary. "Our fields look good, and the competition is big-time," says Cooke.

Bat rules are dizzying. The familiar yellow plastic style - also invented in 1954 - is legal but rarely used. More sophisticated bats - the Ledge Sledge, the AirMax, the Loco Bat - have taken its place. Isenberg has the Rolls-Royce of wiffle bats, a $100 Moon Shot: eight ounces of graphite, guaranteed for life against nicks and dents. Most bats between seven and 12 ounces, unless home-doctored, are allowed.

On the offensive side, a fair ball no one catches cleanly (gloves prohibited) is a single, advancing each "runner" one base. A ball that hits the right or left field fence (80 feet from home) is a double. Off the center field wall (100 feet), it's a triple. Anything over the fence - a relative rarity - is a home run.

The league uses a regular baseball count: four balls for a walk, three strikes for an out. "This way you learn to work the count," says Cooke, "and pitchers have to do the same. It's more strategic." The "strike zone," Big Johnny, is a tin rectangle behind the plate, painted with the image of a Johnny Bench-like catcher. Fifteen inches off the ground, it's 25 inches high, 21 inches wide. Any pitch that touches it is a strike. Calls in the field are reached by consensus.

Every team is guaranteed four games a tourney. The four with the best records advance to a single-elimination playoff. The last team standing wins $300, usually enough to cover expenses, and eight points in the league's Power Rankings.

The USPPBA includes four regions: New England, the Northeast, the Southeast (mainly Florida) and the Southwest (greater Los Angeles). Each has five tournaments a year; all tourneys are played at the region's hub city. By the end of those five tourneys, the team with the most points in each region moves on to the Final Four.

This year's finals take place next month in Sarasota, Fla. - unless, Chrystie says, the film crew now documenting the league is free to shoot that weekend. Then the site would be Philadelphia's, the most photogenic of all the league's fields.

Which brings us to the USPPBA's long-term goal: to gain enough media exposure to generate more revenue, all of it to be spent on equipment, advertising, travel funds and permanent ballparks. It's not far-fetched: the Washington Post, the Boston Globe, Atlantic Monthly magazine, ESPN and Fox Sports have all covered the USPPBA. That might be just a beginning.

"Most board members want a TV contract," says Cooke, himself the youngest elected member. "It's possible, but it's a long way off, 10 or 15 years. Right now, we're just expanding the game. We're already the big leagues of wiffleball, and we want to keep on growing." Cooke knows of 50 to 100 "elite teams," he says, but feels the best is out there somewhere, off the radar screen, in Oklahoma, Texas or Baltimore.

"Can't wait to find them," he says. "That's really what we play for."

Isenberg's aims are just as long-term. Asked what he'll be doing in his 40s, the University of Maryland psychology major laughs.

"I don't know," he says, "but I do want to be playing wiffle, and with these same guys. I want to build that history. How about professional wiffleball pitcher?"

The closer

If there were such a thing, Isenberg might just become the first. Today, New York State of Mind, a long-established juggernaut, is the Stompers' title-game foe. Eight hours in soggy, 90-degree heat has enervated most, but Isenberg summons energy. His sinkers and risers, swerving in and out, batter every square inch of the strike zone. Some miss the plate but hook so sharply they nick Big Johnny for strikes. "Best game I've ever seen Danny pitch," Cooke says later.

New York's pitcher - head shaved, goatee a mess - is one of the toughest in wiffleball. But Gaithersburg sees he has only two pitches today. In the fifth, he wearies and walks the bases loaded.

Cooke hasn't hit much today, but he's studied every at-bat. He expects the drop ball and gets it. He launches it off the top of the wall, inches shy of a homer. It clears the bases. Stompers lead, 6-3.

In the last of the sixth, the final inning, Isenberg starts to tire. He gets the first two outs, then, cursing himself, walks the bases full. His mechanics are off. His eyes search the bench for help.

Schaefer is on the edge of his seat. "I want the last out," he tells Cooke, who simply nods. The righty strides to the rubber and takes the ball.

The drop pitch is ready; Schaefer can feel it. Ten years of muscle memory tell him so. He aims at the top of the backstop and fires as hard as he can. Three times, plunging like an anvil, it rattles Big Johnny. Stompers win.

Before this year, the team had never won a tournament. Now they've bagged three out of four and, for the time being, second place in the nation. "We've snuck up on everybody," says Cooke with a chuckle. "They're not used to us. We're a fluke."

He gets the pleasure of this little kids' game. Sure, it's fun - always has been. But it's more fun now. When you win, he knows, it's never by mistake.