Kirby Lee-USA TODAY Sports

The San Francisco 49ers have developed the reputation of being a physical football team under Kyle Shanahan, and the head coach has found a more creative way to rip the heart out of opposing defenses in the playoffs. 

Put Trent Williams in the backfield.

Shanahan has lined Williams up at fullback and the All-Pro left tackle went motion twice in a formation he's used in his playbook for a decade. Instead of having six-time Pro Bowl fullback Kyle Juszczyk at fullback for the play, Williams has gotten the call. 

"Just one more piece we can add to it," Shanahan said Thursday. "Trent mentioned it to me a long time ago kind of halfway joking. And he was the only guy left and he's probably the best guy you can imagine ever doing it. I can't believe it's legal. It's scary for me to even watch."

Shanahan has even considered giving the 6-5, 320-pound Williams the football to create more nightmares for opposing defenses. The 49ers already have the most physical running game in football with Eli Mitchell and Deebo Samuel, so why not add Williams to the mix this late in the year? 

Williams didn't practice Wednesday and Thursday due to his ankle injury, but Shanahan doesn't think lining up Williams in motion caused his injury in last week's win over the Green Bay Packers. If Williams is good to play Sunday in the NFC title game against the Los Angeles Rams, expect to see him lined up in motion again. 

"It's not something that Trent had to practice too much," Shanahan said. "He had to just time out the motion with the snap count and run full speed through the D-gap and clean up whoever shows. And when they showed, they got out there pretty fast. 

"I thought it was was one of the cooler plays I'd seen, just because of who was on it, not because of the play we ran."