The two leather cover regulation baseballs are connected by a 2 nylon rope. When the arm moves properly through the arc of the delivery, the top ball travels without touching the thrower. When the player starts to short arm the ball, the top ball will flop over to hit the back of the hand. Instant feedback! Can be used for over-hand and side-arm delivery. Great for any player at any level. When you throw and the ball hits you in the hand, it means that you are not using the proper arm arch and that your entire body is not being used properly. Drill sheet included.