Boarding is a penalty called when a player pushes, checks, or hits another player violently into the boards. The key word is violently. Body contact along the boards happens on almost every shift and is legal. Boarding is called when the hit is excessive, when the player being hit is in a vulnerable position, or when they are hit from behind.
This is one of the more dangerous penalties because players can get seriously hurt when they go into the boards head first or at an awkward angle. It usually results in a two-minute minor but can be a five-minute major if the hit is bad enough.
