After winning with the ♠K at trick two, declarer could see that he would always make 12 tricks as long as he could make five tricks in diamonds. So, declarer led a low diamond to dummy’s king and the ♦2 back to his ace.
West’s discard was an unpleasant but not unexpected sight, leaving declarer with the problem of making five heart tricks. East had started with six spades and four diamonds. Declarer knew that he could go with the odds and cash the ♥K and then lead the ♥10, intending to run it if it was not covered – but that would lead to an embarrassing situation if East had been dealt a doubleton heart jack. Instead, declarer decided to complete the count on East’s hand by cashing his three club winners. After East threw a spade on the third round of clubs, his original shape was known by declarer to be 6=1=4=2.
Accordingly, declarer led the ♥10 with the intention of running it if West played low. In practice, West covered with the ♥J and the trick was won by dummy’s ♥Q. Declarer continued with a low heart from dummy to his king and claimed 12 tricks: a spade, five hearts and the six tops in the minors. You should note that if East had an unlikely singleton in clubs then the hearts would have been 3-2, guaranteeing that declarer could take five tricks in that suit. While this line would have lost to East holding a singleton ♥J, it picks up the other four singletons with East. The full deal: