A play fit for a king
Assume you’re East, defending against four hearts. Partner leads the queen of clubs, which wins, and continues with the jack, which South ruffs. Declarer crosses to dummy with a trump and returns a spade.
If you follow low, South makes the contract. His jack forces West’s ace, and there is nothing your partner can do to harm declarer. For example, a spade return to your king would be futile, since it would make South's queen a trick and allow him to discard a diamond from dummy on it. A diamond return would be equally fruitless, while a passive club return would also do nothing constructive, since declarer could develop a spade trick himself by later leading a spade from dummy.
But if you rise with the king when the spade is led from dummy at trick four, and then shift to a diamond, declarer bites the dust. He loses two spades, a diamond and a club and goes down one.
The question that arises, of course, is how are you supposed to know that you should go up with the king of spades at trick four? It seems too unusual a play to make without seeing all four hands; furthermore, it violates the rule of second hand low.
Yet the play of the king is entirely logical. Once declarer ruffs the second club, it becomes clear that the only real hope of scoring four defensive tricks is to assume that your partner has the ace of spades and a diamond trick.
If declarer has the A-Q of spades, your king is dead whatever you do, so you might as well play the monarch and hope for the best. When the king holds the trick, returning a diamond just adds the icing to the cake. ¦