Do not go into your service follow-thru with a straight right elbow ...you need to stop the arms forward motion with the elbow bend as depicted ...elbow bends during pronation ...it is a timing thing that you must work out thru practice ...correctly timing the elbow bend brings the ball down into the service box (along with spin) and also increases velocity via a kenetic 1 - 2 punch (like a flail or noonchuk)
