Steer partner with your cards
Benjamin Spock said, “In automobile terms, the child supplies the power, but the parents have to do the steering.”
In automobile terms, the bridge dealer supplies the cards, but the players have to do the steering; and often one defender must steer his partner in the right direction to defeat a contract. In today’s deal, how should East set his GPS to tell partner how to beat four spades after West leads the diamond three, and South takes the trick with dummy’s ace? In the auction, South judged that his hand was too strong for a weak jump overcall, despite the unfavorable vulnerability. (Discuss his hand with your partner.) Then North plunged into game. He might have taken things slower by first cue-bidding two diamonds to show spade support with at least game-invitational strength, but he could not see how that would help, and the vulnerable game bonus was a powerful lure.
When you have a sequence of touching honors and cannot win the trick (because either someone has already played a higher card than your best or you are discarding), you typically play the top of your touchers -- here, the king. However, if East does that, when West wins the next trick with his spade ace, he will lead a second diamond. Instead, East must give West an incorrect signpost by playing his diamond queen. This in principle denies the king. So, when West is on lead, he will see no purpose in playing a second diamond and will surely shift to a club -- exactly what East desires.