N-S do well to reach 7♥. North’s 3♠ said, “Good hand with Heart support,” South’s Redouble showed a control in Spades, 4NT asked for Aces, and the 5NT response said, “Two Aces and a void.” That void was clearly in Spades, so North took a shot at the grand slam.
Trusting that the ♠A was not cashing, West led a trump, at which point Declarer could see 12 top tricks. The 13th might come from the Club suit (if the Jack would come down in three rounds), or, if trumps were 2-2, then the 13th trick would come from a Club ruff in Dummy. As you can see, both these lines of play were doomed to failure, but there was another line available, in the form of a so-called dummy reversal. Instead of counting six trump tricks in her hand plus maybe a single ♣ ruff in Dummy, Declarer reversed her thinking and went for no fewer than four Spade ruffs in her own hand.
Here’s how the play started: Opening trump lead won in Dummy, Spade ruff, trump to Dummy (East showing out and putting paid to any ideas of a Club ruff in Dummy) and another Spade ruff. Back to Dummy with a Club, another Spade ruff, back to Dummy with the ♦J and a fourth Spade ruff. Declarer had negotiated all the required ruffs but still had to get back to Dummy to draw that last enemy trump. Because there were more Diamonds out than there were Clubs, Declarer reckoned that returning to Dummy with Diamonds was less likely to run into a defensive ruff. And so it was, with the dummy reversal bringing in 13 tricks.
P.S. If trumps had been 2-2, then Declarer would have abandoned the dummy reversal, counting up to 13 tricks with six trumps plus six side-suit tricks plus one ruff in the short hand. But when the dummy reversal became necessary, the arithmetic changed to only three trump tricks (in Dummy) plus six side-suit tricks plus four ruffs in the long hand.