The St. Louis Blues' Ryan Reaves KO'd San Jose's Matt Tennyson Monday and will likely face supplemental discipline.No two questionable hits are created equal. Some straddle the line between acceptable and illegal. Others are just plain ugly and, in theory, easier for the NHL Department of Player Safety to evaluate. The devastating blow St. Louis Blues right winger Ryan Reaves dealt San Jose Sharks defenseman Matt Tennyson Monday night? File it into the latter category. Here's a look at the hit from behind, for which Reaves received a five-minute boarding major and game misconduct: The NHL's definition of a rule 41 violation, a.k.a. boarding: A boarding penalty shall be imposed on any player who checks or pushes a defenseless opponent in such a manner that causes the opponent to hit or impact the boards violently or dangerously. The severity of the penalty, based upon 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 applying the check to ensure his opponent is not in a defenseless position and if so, he must avoid or minimize contact. However, in determining whether such contact could have been avoided, the circumstances of the check, including whether the opponent put himself in a vulnerable position immediately prior to or simultaneously with the check or whether the check was unavoidable can be considered. This balance must be considered by the Referees when applying this rule. Tennyson hits the boards extremely violently, then strikes his head on the ice. One or both of the impacts knocks him unconscious. Reaves hits Tennyson at full force even though Tennyson appears to have no idea Reaves is there. Note that Tennyson glances left, not right. Reaves approaches from Tennyson's blind right side. His back is turned long enough that Reaves has time to avoid the contact, too.