I have heard and used the term overlap differently between pitching and hitting.
As Ken B stated, in pitching, Java and others have referred to overlap as the arm going back while the body goes forward.
In hitting and overhand throwing, overlap is often referred as the hips turning forward as the upper torso resists turning...
Hey J,
Teach a kid to turn the hips and resisting upper torso turning won't always create the same results... and often creates swings where the batters hips aren't turning into and through contact. The hips turn as a result of good posture and front foot orientation/transfer, IMO.
Point being... positive and negative moves create the stretch that optimizes the work the body performs. Best, ~JS