In that model, as the swing is initiated, the direction of force of the top hand is directed toward the pitcher. As the batter turns and drives with the top hand, having the elbow already down places the arm in a good driving position for these mechanics at the start of the swing. Dipping of the back shoulder as the front rises is part of the mechanics of a swing with too much of an uppercut. In order to help a player rid himself of these bad habits and swing with the shoulders and arms on a more productive plain, we must first have a good understanding of what is occurring in his swing that is causing the back shoulder to dip. With many players, the top hand is by far the most dominant hand and he feels more powerful when the hand is lowered so that the forearm is in a more horizontal position for driving.
Weaker hitters lower the forearm to horizontal much earlier in the swing than do better hitters.
In the process of lowering the arm to horizontal, the whole backside drops as he positions his body to turn and extend the arms. The opening of the front side is also more of a body-positioning move rather than to generate body rotation. It is that early rolling hand action that accelerates the bat head in too much of a downward direction causing the dreaded looping or uppercut swing.