The jab opened a cut on his opponent’s nose. Like any good fighter, Chris Marks then attacked the wound with a series of blows that sent blood and sweat spewing from his adversary’s face. Some 700 spectators cheered from the sticky floor of a hot, dark bar, and after three rounds, Marks was declared the victor.
A Boulder dance club isn’t a place you’d normally find a boxing ring, and you definitely wouldn’t expect to see entrepreneurs and venture capitalists—more accustomed to trading deal points than uppercuts— brawling there. But that was the scene on a recent evening as Marks, 46, of Blue Note Ventures, and others put their dukes up for a charity fundraiser called Founders Fight, part of Startup Week, a series of events for entrepreneurs.
It’s an experience Marks never anticipated. He got talked into participating after going to a gym to take his frustrations out on a punching bag after a deal went south. Fighting in front of peers, he says, is intense: “There’s a survival instinct that kicks in. When someone is trying to punch you in the face, you’re not thinking about anything else.”
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The fights—nine in total— were conceived by David Mandell, CEO of PivotDesk, a marketplace for temporary office space. Mandell fell in love with boxing after the grueling workouts helped him get in shape, improve his bad back—and become more productive.
His partner was Carrie Barry, a former national boxing champion turned gym owner. She trained the amateurs, shouting as they threw medicine balls, did push-ups, practiced footwork, punched—and endured black eyes, bruised ribs, sore jaws, and, for some, tears.
“Everyone has a plan until they get punched in the face,” as former champ Mike Tyson likes to say. What can you learn inside a boxing ring?
Mandell and Barry plan to roll out Founders Fights to more cities. “As twisted as it sounds,” says Jonathan Strauss, founder of photo app Luncher, “it energizes you.”
It’s also terrifying, says Nicole Glaros, chief product officer of startup accelerator Techstars. She volunteered to fight, thinking it was just a silly exhibition. When she found out it was serious, she wanted to back out but felt she couldn’t. Glaros lost her match but called the experience “transformational.” As she puts it, “Your fear factor is through the roof. When every cell in my body tells me to run and I still get in the ring, it’s very powerful.”
A version of this article appears in the August 1, 2016 issue of Fortune with the headline “VC Fight Club.”