Remembering the bidding is imperative for best Declarer play.
You hold: ♠K10953 ♥A7 ♦954 ♣K82. LHO (Left-Hand Opponent) opens 1♦. Your partner overcalls 1♥ and RHO makes a (negative) double. You bid 1♠ anyway since you have points and heart tolerance. LHO bids 2♣ and your partner raises to 2♠. RHO bids 3♣ and you pass (downgrading your ♣K). Partner raises you to 4♠.
The lead is the ♥2 and dummy shows up with: ♠AQ76 ♥K10543 ♦K832 ♣—. Partner has upgraded her hand with the club void. Several important points (and inferences) based on the bidding: RHO has four spades to the ♠J and LHO is void in spades (which helps explain their aggressive bidding with only 18 HCP between them). LHO bid both clubs and diamonds, but did not lead EITHER suit. The most common reasons for not leading a suit you (and your partner) have bid is that you have the Ace without the King in your side's suit or you are leading a singleton. You expect LHO to be 5-4 or 4-5 in the minors with a spade void. LHO should have BOTH minor Aces for the failure to lead clubs or diamonds and his opening bid. Therefore, the ♥2 is NOT a singleton, but a true fourth-best lead.
Count your tricks: five spades in your hand (long trump hand)—you can over-ruff RHO if he ever ruffs in with the ♠J. If you can manage three club ruffs in dummy, that is eight tricks. Two top hearts (RHO has two hearts based on opening lead) brings your total to 10 tricks. There is however, no law against 11 tricks in matchpoints. Take the ♥A and lead a low diamond toward dummy. LHO will assuredly hop with the ♦A. If LHO has four diamonds and five clubs, you'll be able to cash the ♦K before embarking on your cross-ruff. If LHO has five diamonds and four clubs, he'll have to read his partner's diamond as a singleton in order to give RHO a ruff. If that happens, you will lose two diamonds and a diamond ruff, but you can still take 10 tricks.
LHO does hop with the ♦A and his partner's singleton ♦Q falls revealing too much. Now, LHO leads the ♦J. You MUST cover with the ♦K even though it will be ruffed. (If you duck, RHO will have the opportunity to discard his one remaining heart and you will go down. RHO can ruff the ♦K and will return either the ♣Q oróbest defenseóa trump. Win the trump cheaply in hand while LHO shows out (as expected), discarding a diamond. Ruff a club on dummy. Cash the ♥K and ruff a heart with a low spade. (If RHO ruffs, you can overruff. Now that RHO has shown a singleton diamond, you know his EXACT distribution is 4-2-1-6. He won't be able to throw away enough clubs to stop you from ruffing three clubs in dummy. So, you take the 10 tricks you counted in the beginning. [After you get three club ruffs in dummy and two heart ruffs in your hand, dummy's fifth heart will be good, so you get style points for discarding your losing diamond on the good heart while RHO ruffs it.]