if the swing is ground up, the hips turn the middle (drag) .. If the swing is middle out, the core turns the hips. The hips resist this turn which creates a deceleration of the trunk to catapult the arms/hands and bat. A funneling of energy.
So If one is closed or stays square with the feet and hips while getting FYB the middle can power the swing.
So If one is closed or stays square with the feet and hips while getting FYB the middle can power the swing.