Known as a rocking cannon in British terminology. A type of nurse used in carom billiards games. With one object ball frozen to a cushion and the second object ball a few inches away from the rail , the cue ball is gently rebounded off the frozen ball not moving it, but with just enough speed to meet the other object ball which rocks in place , but does not change position . Developed to thwart the restrictions emplaced by the Parker's box .