Few people have ever choked Mark Zuckerberg. Khai Wu is one of them.

A professional fighter known as The Shadow, Wu has a black belt in jiujitsu and has been using it on the Facebook founder during private mixed martial arts training sessions the last several months. Their sparring bouts include an occasional down-on-the-mat Brazilian jiujitsu chokehold. “He’s never like, ‘Don’t do that,’” Wu said. “He’s actually asked, ‘Can you give me a little more resistance, a little more force? I want to feel it.’”

Wu’s gym, the Guerrilla Jiu-Jitsu Academy in San Jose, Calif., has been seeing a lot of tech guys like Zuckerberg coming in to learn to fight. “We call them jiujitsu nerds,” he said—people who don’t look like much of a threat until they get on the mat. Zuckerberg, a slight 5 foot 7 inches, may not inspire fear in the octagon, but his strength is in his mind. “You’d never expect these guys to be able to take you down. Next thing you know, they’re attacking you with these extremely technical moves,” Wu said. “You don’t know this nerd is a silent killer.”