A gentle tap of the cue ball with the intention of getting it as tight as possible behind an object ball , in the hope of a snooker . It is most common in the game of snooker , and is often results in a foul in many pool games, in which on every shot , after the cue ball has contacted a legal object ball (a ball-on, then either any ball must contact a cushion or any object ball must be pocketed (potted). A roll-up can be legal in such games when the object ball used for the tactic is very close to a cushion , so that either it or the cue ball lightly touch the cushion after ball-on-ball contact.