South dealer. Neither side vulnerable
Opening lead — king of diamonds
Defense is mostly a partnership project. In the vast majority of deals, the defenders achieve their optimum result by working closely together as a team and guiding each other carefully toward their common goal. Consider this deal where thoughtless defense allows declarer to make four spades while thoughtful defense defeats it.
Let’s say West leads the king of diamonds and East signals with the eight. When West obeys his partner by leading another diamond, the contest is over. South ruffs, draws trumps and finesses the queen of clubs East wins, but South makes the rest of the tricks, depositing his heart losers on dummy’s clubs.
Now let’s suppose that East is more alive to the situation, in which case he overtakes the king of diamonds with the ace at trick one. He can easily deduce from dummy’s high cards and his own that West almost surely had six diamonds for his overcall, so continuing diamonds would serve no constructive purpose.
Pursuing this thought, and recognizing, that hearts offers the only real hope for the defense, East shifts to the deuce of hearts at trick two. If South follows low, West wins with the Jack, continues with a heart, and declarer eventually goes down one, losing two hearts, a diamond and a club.
Once East appreciates the futility of a diamond continuation at trick two, he is forced to pin his hopes on the possibility of his side scoring two heart tricks. Accordingly, he does not signal at trick one, as he normally would, but overtakes the king of diamonds to try to steer the defense in the winning direction.
Playing the ace of diamonds on the king may seem unnecessarily dramatic, but the situation calls for active defense, and East should see the need to take command of the situation.