It's a bit of a calculated gamble by South. There is a 50-50 chance of a Grand Slam!

South has a void in the opponent's Heart suit.

From the Key-card Blackwood (4NT, and then 5NT) bidding, only one ace is missing, and this would be dangerous if it is in Spades. However all the Kings are present.

The bidding has gone beyond 6 Clubs, so a calculated gamble can be made on 7 Clubs!

West leads the 5♥ , Dummy 3♥, and East hopes to win with the A♥ , but South ruffs it with the 4♣ .

The wins with the A♦, and discards 4♥ in Dummy. Then South leads the4♦  which is ruffed by 9♣ in Dummy.

Dummy leads the T♥  which South ruffs with the 6♣.

Next South leads the 10♦  which Dummy ruffs with the 10♣.

Finally two Spade tricks, one Heart tricks, and five Club tricks remain to make the Grand Slam. How lucky is that!