>>> The shoulders used incorrectly are a great source of energy leakage....of "slop". Energy used by the shoulders rotation, does not get to the barrel. At least not as much and not as quickly.
Two things happen in the swing. The hands (forearms) turn the barrel and the hips rotate. They work independently against the shoulders and then sync at or near contact. The shoulders fight/resist rotation to provide leverage that allow both of those mechanisms to work more efficiently. As they do their work against the fighting shoulders, a large amount of energy is built up which eventually defeats the shoulders and the swing is highly energized.
Without shoulder rotation resistance their is no cusp. Both the hands and the hips work against the shoulders. They do their thing independently and concurrently, and a point is reached that something has to give.....the shoulders can no longer hold.....and the cusp is created. <<<
Just to make sure I understand your position. Let us discuss the role of the forearms separately. First the lead-forearm. I would say the rotation of the lead-shoulder about the spine pulls the lead-forearm around. Would you agree or disagree with this.