Is it an active movement of that leg pulling itself forward or should it's movement be a byproduct of the drive itself? I know some instructors teach that pulling the rear leg forward to be a conscious effort.
The back leg is important for multiple reasons. 1. It's your push or thrust leg. Then the body gets open and the back hip/leg should follow the ball. Think of it as though your hip is pushing the ball. That mindset will help to not close the door, hold the leg back disallowing power. I'm trying to think of a part of the body that isn't involved with each pitch. To get your maximum power, the entire body is necessary. Except maybe the appendix. Nobody knows what that's for anyway.