Backstory: Why walk when you can fly?

Rui Cho, a chatty teen, is preparing to pounce. He cuts a lonely figure atop a six-foot-high wall, his knees bent and fists clenched, rocking gently back and forth as he contemplates the five-foot gap between this wall and the one he's about to - I hope - land on. There's nothing to cushion his fall, if it should come to that: just unforgiving pavement that will not treat this wiry kid kindly.

Are you mad, I ask?

"I know," he says, through a mischievous smile, clearly anticipating objections forming in my imagination. "This is more than a sport. It's a way of life," he offers. He's wearing nothing but a T-shirt, jeans, and sneakers. A helmet or kneepads would only "ruin the moment," making the jump "too safe," he says.

There's nothing safe here; he's about to take a leap I can only imagine taking if I was, say, fleeing a raging fire. But Rui's not fleeing anything; he's going to fling himself across this space for sport (sorry, as a way of life). He launches, clawing the air with a peculiar grace, legs reaching toward the target wall. I fight the temptation to close my eyes.

* * *

It's a crisp, clear Saturday morning in a public square on the south bank of the Thames. Normally buzzing with traffic, the square is empty and silent, save for Rui and his friends and the dull thud of sneakers on stone as the teens leap from wall to wall and run precariously along low building-ledges.

The square has ramps, paths, a monument, interesting nooks, crevices; the kind of area once overrun by baggy-trousered skateboarders, who transformed city-spaces like these into stunt parks when workers and bosses went home for the weekend. Today it's the property of a new kind of urban outfit: "free runners." The craze has swept Europe, its rising numbers of young practitioners ricocheting off walls, somersaulting off phone booths, and jumping the gaps between buildings.

Why walk through a city when you can "fly" over its railings and rooftops?

* * *

"See? I overcame danger!" beams Rui after landing safely, and standing perfectly still, on the opposite wall. I resist pointing out that he created this "danger" by climbing up the wall in the first place.

Free running, or "le parkour," its original French name, is all about overcoming obstacles. The sport (that's what I'm calling it) originated in the Paris suburbs in the '90s, when founders David Belle and Sebastien Foucan took the games they'd played as bored teens - ninja-style prancing on rooftops or imitating Jackie Chan - and christened them a sport. It's a ragbag of gymnastics, rock-climbing, and stuntman daring, with a little bit of philosophy thrown in for good measure. (Mr. Foucan has described parkour as "an evolution of the mind.")

The aim is to "flow" through the city, without stopping or pausing and never going backward. And if you encounter an obstacle, such as a wall, railing, or concrete bollard? You do what Rui did: overcome it. It's a strange sport to behold. There are no teams or scorecards; it's not a competition to see who can "win." Most strikingly, there's no audience to watch and cheer, except those bystanders who, by chance, spy the free runners as they traverse the city.

What, exactly, is the point?

"It is about you, the individual," says Richard Sheehan, a tall, energetic 15-year-old who has just executed a "360-degree cat," a leap five feet in the air - feet first - over a railing while turning a full circle. "It's about testing your own limits," he says.

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