This past weekend, I went to an IHS hockey game, and noticed that @SJamison was able to completely send his opponents off of their feet without even applying too great a force to the opponent. Skylor continually used the hip check, which seemed effortless compared to body checking and a lot less painful for the defender. What is the physics behind hip checking? By applying a force further from a player's center of gravity, a defender applies a torque to the other player, causing that player to experience a rotational acceleration which makes it easy for that player to lose his/her balance. The defender's hip does not need to apply a force other than letting the offender skate into their hip basically. By causing a rotational acceleration of any magnitude, an offender can easily lose their balance. Body checking is much more difficult, however, because in order to stop a player's momentum, the defender needs to have a momentum at least equal in magnitude and opposite in direction.