ESTABLISH WINNERS WHENEVER YOU CAN
Francois, Duc de La Rochefoucauld, a French author and moralist who died in 1680, wrote, "To establish oneself in the world, one does all one can to seem established there already."
That's all well and good in front of the impressionable, but not much use at the bridge table. You may claim that, say, dummy's diamond five is established, but if an opponent holds the six, he knows better. However, establishing low cards, turning them into winners, is an important aspect of the game. Using your trumps during that process is often a critical component.
How did South make four hearts in this deal? West started with three rounds of spades. East ruffed the third and shifted to a club.
Yes, three no-trump would have been less trouble, but finding that contract is easier said than done.
Having lost the first three tricks and faced with a club loser, declarer had to establish a third diamond winner. If the diamonds were 4-2, not 3-3, that would require ruffing twice in hand, and South would need three dummy entries -- two for the ruffs and one to return to the board to cash the established winner. Here, these had to be one diamond and two hearts. So, after winning trick four with the club ace, South could afford to draw only one round of trumps using an honor from hand. Then he played a diamond to dummy's king, cashed the diamond ace and ruffed a diamond high in hand. He crossed to dummy with a trump, ruffed another diamond in hand, returned to dummy with a heart and cashed the established diamond five, discarding his club jack.