When declarer leads towards an honour in dummy he gives second hand a problem. Here South plays in 3NT after bidding diamonds.
West leads his fourth highest heart and declarer tries the jack from dummy – an exception to Second Hand Low is when you have just one chance to score a doubleton honour. Unlucky: East had the queen of hearts to beat the jack. Fearing a spade switch, declarer wins his ace.
To make his contract declarer must establish diamonds. He would prefer to lead the suit from dummy, but he cannot afford to give up control of spades by using his only entry. So he leads a low diamond from hand. What should West do? It is tempting to grab the queen and clear hearts, hoping that the king of spades will be an entry to make them. After all, if declarer has ace-king of diamonds this is his only chance to make a diamond trick – he would feel really silly if the jack scored and declarer smugly cashed five diamonds. But on this layout, taking the queen is an error. Partner’s king drops under the ace and declarer makes four diamond tricks and his contract. Playing low allows partner to win and clear hearts, and your queen of diamonds remains as a sure entry to cash them.
How can you guess what to do? Unless you are sure of beating the contract by taking your trick it is best to follow the rule smoothly. We all feel silly from time to time.