Tennis Serve Bullet designed to serve technique development and coming in two sizes: small and big. Go to Sizes Chart to see all device sizes. A wrist is the speed, direction and power of a serve. A wrong serve technique brings muscles over tension, joints dislocation and eventually injury. A right technique makes a serve simple and successful. A wrist action is the key of a serve technique. Tennis Serve Bullet will separate a wrist action from other movements and develop a powerful, fast and rapid wrist stroke.