After you open 1♥ and partner responds 2NT, Jacoby, you show your singleton club, partner bids 3♦. You bid 4NT, Keycard Blackwood, and land in 6♥. The opening lead is the ♣Q. Hearts are 1-1. Plan the play.
You have a certain diamond loser and a possible spade loser. When dealing with losers in two suits, it is almost always right to start with the longer combined suit providing the suit is unevenly divided. Given that your diamonds are 4-3, you may be able to develop an extra diamond trick if their diamonds are divided 3-3. That extra diamond winner can be used to discard a spade from dummy thus avoiding the spade finesse.
Win the ♣A, ruff a club high, cross to a trump, ruff dummy’s last club and play the ace and a low diamond. If East follows with the lowest outstanding diamond, play low ducking the trick into West. Holding a doubleton diamond West will have no safe exit and no spade finesse will be necessary. If West has a diamond exit and diamonds are 4-2, you need the spade finesse.
If East doesn’t play the lowest missing diamond, win the king and exit a diamond hoping for 3-3 diamonds (36%). If you get lucky, you will be able to discard a spade from dummy on your 4th diamond. However, if diamonds are 4-2 (48%), close your eyes and take the spade finesse. Sometimes finesses work. In any case you have 68% chance of landing this contract by giving yourself two chances.
The normal way to play diamonds with this combination is to duck a diamond and then play the ace-king. This technique allows you to retain control of the suit when the suit breaks 4-2. However, you can’t afford to do that here. If you duck a diamond and East wins and returns a spade, you have more or less painted yourself into a corner. You don’t know whether diamonds are breaking 3-3 so you don’t know whether to take the spade finesse or gamble on the diamonds being 3-3. Why put yourself in such a predicament?
This hand is more or less the exception to the general rule of ducking a trick early with this diamond combination because you must discover how the diamonds break immediately because of the threat of a premature spade return from East.
The full deal: