Partner leads the ♦A, and continues with a diamond to your king as declarer plays the ♦Q. Plan the play at matchpoints, where your goal is to take as many tricks as possible.

Solution:
Counting declarer's distribution is a good first step. He should have 6 hearts, he has shown up with 2 diamonds, and has less than 3 spades (if he had 3 spades, he would have supported his partner). If he has 1 spade and 4 clubs, this means that partner should have a singleton club. However, with a singleton club, partner would have surely lead it. You can therefore deduce declarer's distribution to be 2-6-2-3. If there is a club finesse, you know that it is working for declarer. Your best bet therefore is to try to score an extra trump trick for your side.

The correct play now is the ♦J, conceding a ruff-and-discard that will not help declarer. Declarer discards a spade from hand, ruffs in dummy, crosses back to the ♣K, and plays the ♥A. You should be alert now, and unblock the ♥Q. When declarer plays another heart, partner is able to win the ♥9, and play another diamond for you to score your ♥J. This precise defense is required to restrict declarer to 8 tricks.

Analysis:
Bridge Baron's double dummy analysis confirms that the recommended play is your only chance to score an extra trick. The analysis pointed out a way in which declarer could counter your defense. When you lead the ♦J, declarer should ruff this in hand, cross over to dummy to lead a heart. If you play the ♥Q or ♥J, declarer ducks this! If you play a low heart, declarer plays the ♥A and a heart. In either case, declarer restricts you to 2 trump tricks.
Par Contract Analysis:
The par contract on this deal actually is 4♠ by North-South.

Bridge Baron deal No : N2490-97034-63419-85784-31096-27738