Think of the initial stretch (load) as posterior stretch and the release (core stretch) as anterior stretch in the s-plane. Rotation is the result. If there is no anterior vertical stretch then the core isn’t optimized. The obliques work vertically and horizontally.
if two legs aren’t used the vertical force component isn’t used. Much less force is applied.
Most good swings the hips will drop a bit.
Yep, you end up bypassing the "down to"