After an errant pass made it all the way to the boards, Smith had to collect it. With Canadiens defenseman Andrei Markov in pursuit, Smith fired a quick shot towards the net. It went through Markov's legs, then through Bruins forward Patrice Bergeron's legs and just missed Alexei Emelin's leg as well before finally soaring over Carey Price who probably never saw it coming.
Here's a closer look at this angle via @PeteBlackburn:
Considering the angle from where Smith shot, the number of bodies he had to get it through and the way it entered the net, this is just a remarkable goal.
The Bruins had been stymied by Price all night to that point. They also were having trouble hitting the net when they did have some open looks. Smith's opportunity wasn't close to as good as some of the chances the Bruins had earlier in the game. That was a heck of a shot.