North leads the deal with . Now, all the other players must follow suit, i.e. play a spade card. East has a spade card, and thus must follow suit by playing . South, however, does not have any spade card, and thus is allowed to play any card he wants. If he desires to win the trick, he can override North's by playing a diamond card (diamond being the trump), for example . If he does not want to win the trick, he can slough any other suit, such as . Let us assume that he plays , overriding North's card. Now, West still has to follow suit, since he has a spade card, and plays . South's trump card, gives him an opportunity to escape following suit, and he wins the trick.