I was wondering about that, too. I couldn't see (apart from a couple of chains at the back) how drive got to each roller, but maybe there was a differential in there somewhere, with one output or the other being braked by the steering wheel, whereupon the other side would speed up. But then the thing would try to pull itself sideways, because the contra-rotating rollers would no longer cancel each other out....
Nice way to knurl snow, though.
Andy