This was the auction when I watched this hand being played in the club recently. Four diamonds indicated either a singleton or void diamond with spade support and the five no-trump response to Blackwood showed two key cards plus a void. Opener realized the void was in diamonds and bid only six spades. The ace and queen of diamonds were wasted values on this hand.
Several pairs reached slam on this hand, but no one made 12 tricks, which I think means that no one counted the tricks to determine how to eliminate four losers and make the slam. Declarer wins the lead with the king and should presume that the heart lead is a singleton and that to lead hearts again would be disastrous. No defender would trump a heart and fail to lead a club without holding the ace, but having the ace in hand would make it easy. Back to declarer's problem: How are we going to score 12 tricks? The east hand has three losing diamonds and possibly two losing clubs. To make a small slam, declarer must eliminate four of those losers. Because of the high spade spots on the board, declarer can trump two diamond losers, returning to the closed hand each time with a trump. For this line of play to succeed means declarer must trump out the two diamonds before leading trump (other than to return to the closed hand). He must be sure to trump with the ace so he will not be blocked on the dummy. Declarer then draws the two outstanding trumps and uses the jack of hearts to the ace or queen as transportation to the board. The two high hearts enable declarer to discard two of the remaining three losers and make the slam. On this line of play, we have modified our trick count to seven spades, four hearts and the diamond ace adding up to 12. Apparently, all the players who declared and went down started with a trump and could not recover. The big lesson available from this hand is: Count your tricks before you play. Develop a plan to eliminate losers.
Why should declarer trump a diamond first? The answer is that even if trumps are not badly distributed, he can count 13 tricks by using dummy's trumps and getting rid of all the other losers on the hearts. Overtricks are like gold in this game.