“Help-defense does not require that you stop a penetrator’s jump shot. (You cannot expect to stop every shot on the court, regardless of how good you are.) Help-defense does require you to get in the penetrator’s way and stop his layup. Therefore, you move into the penetrator’s path as quickly as possible, and you stop him as far out as you can while still staying across from your man (or down lower than your man, never higher or “up,” which allows a pass over your head to the basket).”

From Dick’s book “Stuff”