It is not every day that you are defending a slam and it is wrong to ruff declarer's side Ace.
Declarer arrived in Six hearts, West leads the DQ, East puts on the King, Declarer won and laid down the HA (correct is H to King, and, discovering the break, duckiing a heart, when a diamond can be tapped in dummy or trumps drawn with loss of one trump trick and spades brought home). Now discovering the break, he played SA. Plan defence.
If partner's carding is to make sense, and declarer's bidding is to make sense. Partner has two diamonds and is marked with the trump void. The deal layout you need to worry about resembles this. If you ruff, where is your side's next trick going to come from? declarer is able to reach his hand with CA and ruff a diamond to make his extra trick: 4 spades, DA, Druff, five trumps and Ace of clubs is 12 tricks. To cut his transportation, West needs to hold off till the third round and then ruff in, and play a high diamond. Now declarer is left without resource as, with HK in dummy, trumps at large, and no spades in hand, dummy's spade winners are marooned.