How to learn the biomechanics of the Slice Serve: On SpinPro: The Official Rules are that the serve must be moving in an upward trajectory: "4.A.7.a. The server’s arm must be moving in an upward arc at the time the ball is struck with the paddle". The SpinPro simply teaches you the biomechanics of spin when using the screens. To hit a legal serve, remove the screens and add in a forward swing. This will naturally happen when you are serving on court into the box as you need to lift the ball up and over the net to get it in.