Coined after the infamous Buffalo Sabres enforcer, the rule (Rule #56: Fisticuffs) states that a player shall receive a game misconduct if they engage in a fight and his jersey is not properly "tied-down" (a strap fastening the jersey to the pants), and loses his jersey (completely off his torso).
If the player loses their jersey despite the tie-down being intact they are not issued a penalty.
The rule was enacted after Rob Ray would intentionally remove his jersey, elbow pads, and shoulder pads in order to gain an advantage in a fight. Removing the equipment had two advantages, it made it easier to throw punches, but also gave the opponent nothing to grab on to during the altercation.
The other reason for enacting the rule was to put an end to the spectacle of shirtless guys fighting.