When a player uses his stick or leg to illegally upend an opponent.
Violation of the rule is usually a minor penalty, however, if injury occurs a 5-minute major could be assessed. Tripping usually occurs when a player uses his stick to pull the legs out from an opponent but it also happens sometimes by accident when a player is poking at the puck but hits an opponent's skate.
Tripping is one of the most common penalties that lead to penalty shots being called. One of the famous penalty shot tripping calls occurred in the 1994 Stanley Cup Final when Brian Leetch hauled down Pavel Bure in Game 4. Bure did not score on the penalty shot and the Rangers went on to win the final.