West leads K against 3NT, South executes the Bath coup by ducking, and West must switch. If he plays a red suit, declarer will play on hearts (and will have to guess the position correctly).
In this bridge scenario, West starts with a King lead against 3NT, followed by South's Bath coup by ducking.  West must switch to another suit if he plays a red card, as declarer will then play on hearts and needs to guess the exact position of the cards.