Hitting Jack-it Weighted Bat Jacket Set (9oz, 12oz & 16oz) Easy to Use; Quickly develops bat speed and power Sold in pack of 3 with sizes 9, 12, 16 oz. Increases Ball Exit Speed Gives you more confidence as a hitter Strengthens hand and forearms Current MLB Head Coach Clint Hurdle, Manager of the 2011 Pittsburg Pirates, fully endorse the Hitting Jack-It Weighted System, for building strength and bat speed. The Hitting Jack-it is a complete hitting training system that comes in four sizes to assist progressive training. The product works with all baseball and softball bats allowing players to develop during live batting practice sessions and off hitting tees.