Knowing what's about to happen, this is a little bit like watching that woman skinny-dip at the beginning of Jaws. It's just not going to end well for her. (This video could do with a suspenseful score to really up the fear factor, come to think of it.)
Smith skates into the corner after a loose puck, and Comeau tracks him all the way across the zone, looking at nothing but Smith's numbers.
That never changes, but Comeau crushes Smith anyway. "As Smith plays the puck up the ice, Comeau finishes him into the boards, from behind, and with significant force, causing a violent collision," explains Patrick Burke.
Burke also explains what Comeau was supposed to do: not this. You can see in the final angle, played as Burke announces the suspension, that Comeau has both hands square on Smith's back as he drives through the Red Wings' defender. That's never going to end well.
Still, it ended about as well as both Smith and Comeau could have hoped. Comeau is fortunate that Smith got up from that, and he's also fortunate that he had no history going into this hit.