On the second round of Hearts partner drops the Queen, bit unfortunate . However, now you will of course play another Heart for him to hopefully ruff (maybe he dropped the Queen just now because he started with QJ8). If he is going to ruff you need to tell him what to play next and you will do this with a suit preference signal and play back your lowest Heart now telling him to please switch to clubs (despite him seeing the Ace in dummy). So many of my students are intimidated by Aces in the dummy and will not play the suit because of it J  However partner notices your suit preference and duly plays back a Club. Declarer wins and draws trumps and then starts on Diamonds. Partner wins the Ace and duly plays a club to defeat the contract by one. Without the Club switch declarer would have the timing to lead up to his Diamonds twice and then be able to chuck the losing clubs in dummy on the established Diamonds.

You defeated the contract by taking 2 Hearts, 1 Heart ruffed, and 1 trick in each minor for a well defended hand.