On defense, when you need to pick up a dribbler and help a team- mate who is being beaten, remember to pick up or help across from your man, not up from him. This is simply done, if your man stays outside or merely stands still while you are helping. But if the man you are guarding is a good player, he will try to get behind you, down low for a pass over your head. Your job, even though you must help, is to stay across from your man, so that you can recover and get a hand in his face if the ball is thrown to him.
If you move across to help and your man moves down toward the basket, then you must move down toward the basket in order to stay across from your man. Don’t permit your man to get behind you and receive a pass over your head. 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).
As long as you remember to keep your man in your sight while you help, you will be in good position. If you lose sight of your man, you are suddenly “up”, not “across”, and you will have to get back fast.
From Dick’s book Stuff
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