Hitting the bottom half = Homeruns/line drives to the fence. Hitting the top half = Ground ball or over spinning it for a weak pop up. If you're consistently doing the latter two, it's because you're hitting the top half. But figure out what's CAUSING you to hit the top half rather than just trying to swing underneath to hit the bottom half. You'll most likely end up swinging too far underneath it if you do that. The Hitting Vault has some really great drills that help teach the correct bat path