Of interest is what the OHL rule book says about boarding (which it borrows word-for-word from the NHL rule book):
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. However, there is also a responsibility on the player with the puck to avoid placing himself in a dangerous and vulnerable position. This balance must be considered by the Referees when applying this rule.
Any unnecessary contact with a player playing the puck on an obvious "icing" or "off-side" play which results in that player being knocked into the boards is "boarding" and must be penalized as such. In other instances where there is no contact with the boards, it should be treated as "charging."
The wording there is interesting. Most hockey fans are familiar with the "two full strides" rule of thumb for charging, but the rule book doesn't say that either; it simply says that "Charging shall mean the actions of a player or goalkeeper who, as a result of distance traveled, shall violently check an opponent in any manner."
Got it? That's why an otherwise clean hit might be illegal. There is such a thing as too hard where body checks are concerned. As a former defenseman, I find this interesting, since when I played meaningful games years ago, equipment and the size and strength of players were such that it would be nearly impossible to seriously hurt someone with a clean open ice hit, to the extent that I saw players knocked out maybe once every couple of years, rather than every week, as it seems to happen now.