The Hitting Jack-It is the only Variable Weighted Hitting System on the market today. With its patented design the products will help develop proper swing mechanics for the hitter. The 9 oz. size develops bat speed, the 12 oz. works on strength and the 16 oz. develops power. All current weighted products on the market today are placed out over the barrel of the bat causing it to become end heavy. Swinging those types of products cause you to cast your bat away from your body developing bad swing mechanics.