Through 10 games, Zwinak has carried a team-high 138 times for 686 yards and four scores. And he's been dropped for a grand total of three yards in losses.
He's a 235-pound guy, Nittany Lion coach Bill O'Brien said. He's strong. He's a punishing runner.
Making the package that much more frightening is the fact that Zwinak is also faster than you might think. He proved that against Nebraska last week, when he took what appeared to be a simple handoff into the gut of the Cornhuskers' defense, ran through the front seven and then outraced the secondary for a 50-yard touchdown.
It was the longest run of his career, doubled PSU's previous long run of the season and was -- get this -- was eight yards longer than former Lion and current USC tailback Silas Redd's career long when he was at Penn State.
In short, Zwinak is a frightening package for opposing defenses. But there is one other thing you ought to know about him, something that belies his on-field persona of bruiser.
I'm always nervous coming into a game, Zwinak said. I'm a nervous kind of guy.
Earlier this season, Zwinak admitted to trying to maintain a low profile at Penn State, even as he was becoming more of a household name on the football field.
I usually try to stay to myself on campus, he said at the time. Just another kid.
Maintaining a level of anonymity didn't appear to be a problem coming out of spring practice. Recruited as a big tailback, Zwinak redshirted as a true freshman in 2010. But he blew out an ACL in practice that year and -- by his estimation -- did not round back into top form until the start of the 2011 season.
With Redd established as the starter, Zwinak carried three times for seven yards in 2011.
Following the spring of 2012 -- even after a coaching change -- Zwinak was still mired at the bottom of the depth chart, behind Redd, converted receiver Bill Belton and former walk-on Derek Day. Most just assumed he was also behind Curtis Dukes, the power back who was sitting out the spring to work on academics.
But shortly after PSU was hit with NCAA sanctions in July, Redd transferred to USC. Belton moved up to the first team. But Belton was injured in the season-opening loss to Ohio. Day moved up to the first team. But he was injured in a Week No. 2 loss at Virginia.
With Dukes never quite grasping O'Brien's complicated offense, Zwinak began seeing more and more playing time. Despite not starting, he had back-to-back 100-yard games in wins over Illinois and Northwestern. He made his first start of his career in Week No. 9, at Purdue, and responded with 134 yards. Then he went off for a career-high 141 at Nebraska.
I came into the season with no expectations, Zwinak said. Everything changes week to week. I've just done what they've asked me to do and played how they've asked me to play.
We had injuries occur, O'Brien added. And as injuries occur, the next guy has to step up. That's what happened to Zach. We had injuries, and he was the next guy.
Something of a nervous next guy, to hear Zwinak tell it. But he said all of the nerves disappear when he takes the field.
Once the game starts, you don't notice it anymore, Zwinak said. It's just playing football like you have your whole life.