The purpose of using a heavy bag is to absorb the bat's energy at the contact point and to train the batter to expend all bat speed-generating energy (rotational and torque) prior to or by contact. Good transfer mechanics and timing will have the batter depleting his rotational and torque energy as the bat-head reaches maximum velocity. Therefore, after all of the body's energy has been transferred into bat speed, the body and limb muscles are at rest. The hip and shoulder rotation is complete, lead-arm pull and back-forearm lowering to horizontal position is complete, and the hand-path has slowed to a stop. There is no energy being applied to the bat after contact. The bat's energy has been expended into the heavy bag. Now you now have a frozen frame of the batters mechanics at contact which can show see if batter is properly rotating into the swing which is an important part of the swing. This also works on power within a swing.