A football can slip out of a running back’s hands in a split second. For Fred Warner, that split second is a skill he has spent hours training for, sometimes with boxing gloves on.

The San Francisco 49ers linebacker has turned punching the ball loose into a trademark, earning the move its own nickname: the “Fred Warner punch.” But the secret behind one of the NFL’s most effective turnover weapons starts away from the football field, where Warner joins fellow 49ers defenders in an unlikely offseason hobby: boxing.

The results show up in the numbers. Warner, who drew the wrath of NFL fans for his "dirty" hits on Monday, has three forced fumbles in six games this season, tied for the most in the NFL, and 20 in his career, the fourth-most in the league since he entered in 2018.

The three players ahead of him are T.J. Watt, Khalil Mack and Myles Garrett, all pass rushers. Warner has just 10.5 career sacks, so his takeaways come from hitting the ball, not from speed.

Patrick Willis, the Hall of Fame linebacker who played for San Francisco from 2007 to 2014, says boxing is a big part of it. “I would say just working out with him in the offseason a couple of times, doing some boxing. A lot of it comes from training it. He trains it, and he does it at a high level. Right now, he’s the best at doing it,” Willis said.

Ji’Ayir Brown, a third-year safety, confirmed the boxing, then added some nuance. He said Warner is not alone in the ring.

“Yeah, me, Fred, we box in the offseason, man. Dre Greenlaw, we all box together in the offseason,” Brown said. The goal is broader than one punch, according to him. “I wouldn’t say just to work on the peanut punch, but we do it more for cardio, man,” Brown said.

Cardio is only the start. Brown said boxing carries over to football in two ways. The first is physical.

“Hand-eye coordination. Hitting a target, you know, when you’re tired,” Brown said. That matters for a linebacker who needs to find a football moving through traffic late in a drive.

The second is mental.

“You’re counting boxing, you know, when you’re throwing your one-two-threes, your combinations. Remembering those combinations when you’re tired, so that helps you out in football, and kind of remembering routes, remembering your keys, formations, all that,” Brown said.

The two ideas fit together. A boxer has to recall a sequence while exhausted, and a defender has to recall his assignment in the same state, deep into the fourth quarter. The punch itself requires timing, because the ball moves at the exact moment a runner shifts it.

Opponents have noticed. After Warner punched the ball out of his hands against the Seattle Seahawks, running back Emanuel Wilson said he would change how he practices.

“When we got our individual period time, just having guys punch at the ball. Everybody really. And I’m going to talk to the defense, the scout team defense, and ask them to just punch at the ball a little more,” Wilson said.

Willis sees a lineage in the group. Asked about Warner and Greenlaw, he said they remind him of himself and NaVorro Bowman. “They’ve been able to take the standard and raise it,” Willis said.