It has been implemented since I was old enough to check. If a player has his head down, hit him. A simple and easy rule to follow.
Nobody wants to see another person get hurt. But in a game centered around physicality, injuries are often going to occur. The biggest debate right now is over shots to the head.
By now, we all know Pittsburgh Penguins star Sidney Crosby is just shy of two months on IR from a concussion. We all know Marc Savard is again sidelined with a concussion, and his return to the NHL seems doubtful.
These are unfortunate injuries, but they are part of the game. The NHL cannot disallow hits to the head that occur by a legal means. By legal, I mean contact that could be avoided if the player protected himself.
Players who put their head down to admire the puck on their stick should be hit. They have full means to protect themselves, and failure to do so will involve a painful realization.
If the NHL were to implement a ban on hits to the head, they will reduce the beautiful game to a drop-in game at your local arena.
Players are apt to avoid being hit by skating around with their head down. I'd hate to see hockey turn into a game where the physicality becomes extinct.
I do not condone hits that could be avoided. It is not part of the game to intentionally injure an opponent.
Yet an all-out ban on hits to the head will be just one step to removing checking all together.