Stanford football fans may have been caught off guard on Friday when, in the midst of a quiet game for the Cardinal’s dominant front seven, a defensive back broke the ice with the first Stanford sack of the 2012 season. But that defensive back, senior Usua Amanam, felt even more puzzled.

“Dumbfounded,” he recalled Tuesday. “I’d say every single emotion was kind of coming at me. But I had to line up and play another play.”

And for Amanam, lining up in the secondary is a bit dumbfounding in and of itself.

The senior, once a 1,800-yard running back at San Jose’s Bellarmine College Preparatory, took on the transition to the defensive side of the ball as the Cardinal prepared for the Orange Bowl in 2010. The squad had a reliable stable of rushers, after all, and was just a season away from losing its three top defensive backs to graduation.

Twenty months later, the new Amanam was unveiled as Stanford’s starting nickel back and was its saving grace in a 20-17 win over San Jose State. Without his four tackles for loss and fumble recovery, the Cardinal could very well be staring at an embarrassing upset and an 0-1 record. (If you’re struggling to pronounce his name—“OOSE-wah uh-MAN-um”—just go with the Nigerian translation of his last name: “job well done.”)

That first sack was a moment of truth for Amanam, who hasn’t made a big play since his lone touchdown reception two seasons ago.

“A touchdown’s one thing, but there’s nothing like getting a sack,” he said. “I’ve always had friends on the defensive side of the ball, and now I really understand what it feels like.”

If his transition — from fifth-string running back to top stopper—seems a familiar trope, that’s because it’s hardly the first of its kind on recent Cardinal teams. Delano Howell switched from tailback to strong safety in 2008 before Richard Sherman followed suit in 2009, going from a promising receiver to Stanford’s most reliable corner and, eventually, an NFL defender.

But the man who reminds coaches of Amanam the most never played offense—at least in college. Michael Thomas hung up the pads as a dual-threat quarterback as a true freshman and went on to become one of the Cardinal’s most electric defenders, playing free safety all four seasons.

“Usua came over as an offensive entity and learned from Mike Thomas,” said defensive coordinator Derek Mason. “He learned how to work, he learned how to practice, he took every mental rep that he could. And when it was his turn to establish his own physical play, that’s exactly what he was trying to do.”

The switch was not instantaneous for Amanam, who admits in hindsight that the possibility of returning to offense—an option left open by his coaches last season—may have held him back a bit. But it’s the same athleticism that helped him exploit gaps in the offensive line for big rushing yards that made him a formidable pass-rusher against the Spartans.

“When I started blitzing in spring ball,” he said, “I realized that I could use some of that ability I had on offense to try to dodge and try to avoid people.”

That ability has made him a great fit at nickel back, a hybrid position of sorts described by Amanam as a mix between corner, safety and linebacker. As the occasional fifth—hence the term “nickel”—defensive back on the field, Amanam may not be in for every snap, and he says that the hardest part of his mental adjustment was not always having the ball in his hands like he would as a running back.

Still, the season opener demonstrated what an impact Amanam can have at a position that fits him so well.

“I like being closer to the action [than a corner or safety],” he said. “I feel more at home in the middle of the defense than anything else.”

“The position started to make sense to him,” remembers head coach David Shaw of Amanam’s development late last season. “I firmly believe he’s going to get his hands on a couple passes this year. I’m excited for him, getting roots in that position.”

Mason, who worked with Thomas and Howell after arriving in 2010, benefited from the leadership of former offensive players in each of his first two seasons coaching Stanford. But he believes that the success of Amanam and his predecessors is not just a product of their athleticism.

“I don’t know if it has as much to do with playing on the offensive side of the ball as it is about finding a home and feeling comfortable with the guys that you’re playing with,” Mason said. “It’s family. Those guys believe in one another, they play hard for one another. You know, that ‘S’ on the side of that helmet means something to those guys. When those guys put that helmet on, they understand exactly what the Stanford credo is.”