You hold: ♠Q965 ♥AQ874 ♦A102 ♣2. Your partner passes. South opens 1♦; you overcall one heart. North bids 2♣; partner passes; South bids 3♣ and North jumps to 5♣.
Partner leads the ♥5 and dummy reveals: ♠J ♥K96 ♦KJ964 ♣K764. Declarer calls low from dummy, and you put in the Queen while Declarer plays the two. What next?
You need three tricks to set the contract. IF your ♦A lives, two hearts and a diamond will do it. If, however, your ♦A does NOT survive, you will need partner to have a doubleton heart so that you can get two hearts and a heart ruff. You also need to worry about the fact that a lot of spades are missing. If Declarer has two top spades, one of Dummy's hearts could go away, and if Declarer has AK10 of spades and risks finessing you, two hearts could go away. So you cannot afford to risk passive defense such as returning a trump or risking a spade.
So, what is the correct order of play? Look at the heart spots. The ♥J, ♥10, and ♥3 are missing. Partner led the ♥5. Declarer MUST have either the ♥J or the ♥10, because partner would have led the ♥J with both of those cards. If partner had ♥J5, partner would have led the ♥J. If partner had ♥105, partner would have led the ♥10. If partner had the ♥J53 or ♥1053, partner would have led the ♥3 NOT the ♥5. So, partner MUST have started with the ♥53 and is desperately waiting for her heart ruff.
Even if you don't go through the full analysis, it is correct to lead a second heart at trick 2 because you KNOW that Declarer has at least one more heart (either the ♥J or ♥10). Partner's second heart will clarify the situation. If partner plays that ♥J or ♥10 on the second round, then you are forced to assume that she led the wrong card from ♥J53 or ♥1053 or ♥J105 and you will have to play the ♦A and hope it cashes. If, however, partner follows with the ♥3, promising a doubleton, you will play back the ♥4 (lowest card for lowest side suitódiamonds—for partner to return after she ruffs the heart). Declarer will be down two.