There’s a tiny storefront with a barber-pole decal on its window tucked into an alley in San Francisco’s South of Market neighborhood--the part of SoMa that's a roughly equal-parts mix of car-repair shops, residences, and start-up headquarters. Inside, there's a bench, a chair for haircuts, and a boxing ring. There are generally also a couple of broad-shouldered guys either sitting around drinking tea or punching one another in the face.

These guys are rotating members of an elite and unlikely club--the fighters in training of San Francisco's start-up scene. They are techies, programmers, and, to a surprising extent, founders and CEOs of companies. The creator of Second Life is a regular; so is the co-founder of CrowdFlower. Each is a student of Michael Onello, who is better known in these parts as Michael the Boxer.

Onello, a tough-talking New Jersey native, says that when he moved his gym from Miami to San Francisco, he expected to be coaching "more women and more gay people--but more entrepreneurs? I didn't expect that."

Students come from Google, eBay, Oracle, and other tech companies. Not everyone returns after the first grueling lesson.

"The entrepreneurs are more likely to stick with it," Onello says, "because they usually are lone wolves. Boxing is not a team sport. You win alone; you lose alone."

Perhaps it's true that a certain type of person is drawn to the boxing ring. Onello has been chewing on the idea for years. "It's the guy who takes the risk in life but also wants the reward," he says. "These are guys who want to be top dog, and a lot of them are." 

Philip Rosedale, founder of Second Life and of the new virtual-world start-up High Fidelity

Stress levels at Second Life were high three years ago, when Rosedale started boxing. "I thought, What could be better than being punched in the face to learn about not avoiding conflict?" he says.

It worked: Rosedale says the intense range of sensations he experiences in the boxing ring has taught him to stay cool during confrontations.

Mitchell Zuklie, chairman of global law firm Orrick and adviser to VC firms and tech companies

Zuklie says he started boxing after noticing a couple of his clients "looked more fit and seemed more comfortable in meetings." They sent him to Michael the Boxer. 

Lukas Biewald, co-founder and CEO of CrowdFlower

"When I'm afraid, I attack," Biewald says. "It's not brave; it's crazy and impulsive. I have the same problem at work." Boxing has taught him to step back. "I think it's been fear that has made me super aggressive," he says. "Now, when I start feeling uncomfortable or nervous, I recognize that."

Here, Biewald drills at the Michael the Boxer gym in San Francisco.

"Challenges in the boardroom seem really small compared to three minutes of someone actually trying to hurt me in the ring," Biewald says.

David Chen, co-founder of NunaHealth, a health care analytics start-up

On a typical day, Chen spends up to 14 hours writing code; he started boxing as much to turn off his brain as to work on his body. "You don't think about code," Chen says.

"You don’t think about work. You think about the best way to hit a bag. It makes me less neurotic." Here, Michael Onello, gym owner and guru, coaches Chen.