As explained by Doug Smith:
Sometimes your team just doesn’t have a spark and they need a little get-go for the night and you need to step up and be their spark plug. You line up next to the guy who’s the other team’s heavyweight and you say to him, “Hey listen, I got to get my team going. Want to go tonight?”
We’ve got a code of conduct, so to speak. That guy might say, “Yeah, no problem. Let’s go, Smitty,” and we drop our gloves and have a good, fair fight. But he might say to me, “You know what, I can’t fight tonight, I got a bad hand.” I’ve had guys say to me, “I can’t fight you. My coach said he doesn’t want me to fight tonight.” At that point, our code says that you don’t jump the guy, you don’t sucker-punch him, you don’t do anything dirty. You just catch him the next time around.
There was a very interesting series on the life and death of NHL enforcer Derek Boogaard a while ago at The New York Times.