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 severitReaves-bad-640x439y 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.

Also worth considering: the NHL’s definition of a rule 43 violation, a.k.a. checking from behind:

A check from behind is a check delivered on a player who is not aware of the impending hit, therefore unable to protect or defend himself, and contact is made on the back part of the body. When a player intentionally turns his body to create contact with his back, no penalty shall be assessed.

Tennyson could not have seemed less aware of Reaves’ presence, making Tennyson utterly defenseless. He didn’t turn his back at the last second.

As a straight violation, Reaves’ hit checks off all the wrong boxes. He’s a likely candidate for supplemental discipline. The fact Tennyson was knocked out threatens to lengthen Reaves’ sentence should the NHL decide on supplemental discipline. Reaves isn’t officially a repeat offender under the current collective bargaining agreement, but he was fined for a dangerous hit on the L.A. Kings’ Anze Kopitar in November, and that incident will be considered as part of Reaves’ history.

It’s tough to imagine a scenario where Reaves escapes without a stiff punishment for this hit. Expect him to sit out at least a few games.