42.1 Boarding – A boarding penalty shall be imposed on any player or goalkeeper who checks an opponent in such a manner that causes the opponent to be thrown violently in the boards. The severity of the penalty, based upon the degree of violence of the impact with the boards, shall be at the discretion of the Referee.
There is an enormous amount of judgment involved in the application of this rule by the Referees. The onus is on the player (or goalkeeper) applying the check to ensure his opponent is not in a vulnerable position and if so, he must avoid the contact.