Swivel Disks help players feel the optimal body rotation to generate powerful topspin groundstrokes and also for the timing and rhythm for serving! Each 11″ slip resistant Swivel Disk has dozens of steel ball bearings to ensure a smooth circular glide. Some coaches call the movement experienced with Swivel Disks the “figure eight” rotation. Whatever you call it, players will gain a quick feel for naturally opening the hips to gain more power from the body’s core. A second benefit is increased loading of ground forces, the result of improved balance.