It's now or never...

This hand, or something very much like it, came up last Monday evening. At matchpoints, both sides vulnerable, sitting West you pick up a decent 13-count:
♠ A x   ♥ K Q 10 x x x   ♦ 7   ♣ A x x x

Dealer South, on your right, opens 1♠ and you overcall 2♥. The rest of the auction: 

South	West (you)	North	East
1 ♠ 3 ♦ 3 ♠	2 ♥ Pass Pass	Pass Pass 4 ♠	Pass 3 ♥ All pass

Your singleton diamond is a pretty obvious lead. Two black aces for entries, maybe partner has the ♥A for an entry to give you a diamond ruff. With visions of +100 dancing in your head, you lay down the ♦7 and see this dummy:

	North
	♠ 9 8 x ♥ x x ♦ A 9 8 x x x   ♣ x x
West (you)
♠ A x  ♥ K Q 10 x x x ♦ 7 ♣ A x x x

Hmm, wonder why North-South aren’t playing in their huge diamond fit? Holy cow, what if you’ve given partner a ruff by leading your singleton?! No such luck; declarer plays low from dummy, partner follows with the ♦J and declarer with the ♦K.  Declarer leads a small spade and of course you fly with the ♠A. Now what?

ANSWER: It’s now or never for the ♣A.

WHY: South must surely have a five-card diamond suit for her 3♦ rebid, vulnerable, across from a silent partner. With six diamonds in dummy, partner’s ♦J must therefore be a singleton, too; she can’t return a diamond for you to ruff. Given all that bidding from South, declarer’s spades are probably solid other than the ♠A; there’s not much of chance for partner to hold a spade trick at this point. As soon as declarer gets back in, she’ll draw the rest of the trumps and then run the diamonds ending in dummy, discarding a round-suit (club or heart) loser if she still has one.

THE PRINCIPLE: At matchpoint scoring (duplicate pairs), there are times to give up on trying to beat the contract and instead cash your sure winners. If declarer has control of trumps and an established side suit for discards, it’s cash-out time. Giving declarer an extra trick is the difference between an average board and a zero! But at rubber bridge or at IMP scoring (duplicate teams), never give up on even the slimmest chance to defeat the contract.

The full deal (hands rotated from their original positions):

	North
	♠ 9 8 x ♥ x x ♦ A 9 8 x x x   ♣ x x
West (you)		East
♠ A x  ♥ K Q 10 9 x x ♦ 7 ♣ A x x x		♠ x x ♥ J 8 x x ♦ J ♣ K J 10 x x x
	South
	♠ K Q J 10 x x ♥ A ♦ K Q 10 x x  ♣ Q

By playing the ♣A at trick three, you hold declarer to making five; otherwise she makes six. Note that  five diamonds is a safer contract for North-South -- since it makes against any diamond split -- but turns out to be an inferior matchpoint contract if four spades is makeable (as it is here).