Hands are from the San Diego Unit Game at Adventures in Bridge, October 21, 2018.
East (Dummy): ♠5 ♥10862 ♦AQ1042 ♣Q84
South: ♠A983 ♥KJ97 ♦K753 ♣6
The bidding is as follows: LHO (West) opens 1♣. Your partner (North) doubles. East bids 1♦—''bid where you live”—your values are in diamonds. Don't bid your lousy 4-card heart suit when RHO (North) has doubled for the majors. South now bids 2♣—a cue bid showing a good hand and asking partner to pick which major she prefers since you are 4-4. West now bids 2♠ and partner (North) passes.
East bids 3♣ and you bid 3♥. LHO bids 3♠ and partner passes. RHO (East) bids 4♦ and West bids 5♣. Partner passes. East passes. You double for penalties. Partner leads the ♣J. Declarer wins the ♣Q on Dummy and cashes the ♦A, discarding the ♥5. Now, Declarer leads the ♠5 from Dummy. Your play?
You MUST duck the spade for a couple of reasons. Most importantly, you do not have another club to lead to cut down on Declarer's ruffs in the short-trump hand. Secondly, where are partner's points for her take-out double? She has the ♥AQxx (which is why she did not lead a heart, although there is an argument for cashing the ♥A on this auction as West has shown at least 6-5 in the black suits and Dummy has bid diamonds). Partner has the ♣J and presumably the ♦J. Even with the ♠K, she barely has 11 HCPs for her double. If your partner has made a super-aggressive double with 10 HCP, she will have the ♠Q, and as long as you duck SMOOTHLY when the spade is led, Declarer is likely to mis-guess, playing partner for the ♠A.
If you duck the spade, partner’s ♠K will capture Declarer's ♠Q and partners can put another club on the table. Declarer will be able to ruff only ONE spade on Dummy. He will end up losing THREE spade tricks because you and your partner have all the spade spots.
If you hop with the ♠A and try a heart, Declarer will ruff. (He is 7-5 in the blacks!) He then plays the ♠Q for a ruffing finesse. Partner covers with the ♠K and Declarer ruffs on Dummy. Declarer ruffs a red card to his hand and ruffs another (low) spade on Dummy. A red suit ruff back to his hand. He pulls trump (only one more from partner) and plays the ♠J. Your the ♠10 falls and Declarer's fifth spade is good. That's +950 to the opponents instead of +200 (you duck the spade) or +500 (partner leads the ♥A and shifts to a club AND you duck the spade).
South (Dummy): ♠432 ♥85 ♦K962 ♣Q843
West: ♠1065 ♥Q64 ♦Q108 ♣KJ7
You (West) pass and North opens a strong (15-17) NT. All pass. Partner leads the ♥2.
You play your ♥Q, which wins. Time to take stock. You know from partner’s fourth-best lead that Declarer ALSO has four hearts. When partner's 4-card suit is Declarer's 5-card suit, it is almost always correct to switch. Sometimes it is also correct to switch when partner has four and Declarer has four in the same suit. How good can partner's hearts be?
Partner does NOT have ♥KJ10x or ♥AJ10x, because she would have led the ♥J. Partner does NOT have ♥AKJ10 because she would have led the ♥A (requesting partner to unblock an honor or give count without an honor). Partner probably does not have ♥AKJx because she might have led the King to look at Dummy. Partner COULD have ♥AK10x or ♥KJ9x, in which case a heart return will wrap up the suit for partner. (However, if Declarer had ♥A10xx, he probably would NOT duck your ♥Q, so the latter holding is unlikely.) So, there is one holding in which a heart return will result in three tricks for your partner.
Could partner's heart holding be really weak (such as ♥Jxxx or ♥10xxx or ♥Kxxx)? These holdings are not possible because Declarer would kill your ♥Q if he started with ♥AK10x or ♥AKJx or ♥AJ10x. If partner has ♥KJxx, Declarer should be taking the ♥Q now to help protect his ♥10 in the suit.
So, there is only one holding partner can have where an immediate heart return by you will garner three tricks. You also expect to get in one more time in clubs (holding the ♣KJx over the ♣Q in Dummy). So, you will have a second opportunity to lead hearts to your partner at that time. If Declarer canNOT beat the ♥Q, then partner has led from ♥AKxx of hearts and definitely does NOT have the ♥J.
So, it appears that it is worth the risk to switch to the ♠10. (Even if partner has the Ace and the ♥K, she may need you to lead a spade from your side of the table so she won't be finessing herself later in spades.) There is certainly room, in terms of HCP, for your partner to hold ♠KQx(x) or ♠KJx(x) or ♠AJx(x).
If you switch to the ♠10, Declarer will play ♠K. Partner will take ♠A and return the ♠7 to your ♠9 and Declarer's ♠Q. [Note that partner's the ♠7 is her LOWEST spade because you have the ♠6 and ♠5 and Dummy has the ♠432. It appears that partner had four spades along with you and Declarer started with ♠KQ doubleton.]
Declarer can take four diamonds tricks, finessing your queen. Partner will discard a low heart (confirming that she started with ♥AKxx originally) and a low club on the diamonds. When Declarer plays a club to the ♣Q, you take the ♣K and return a spade. Partner gets two more spade tricks and a top heart. You end up with three spade tricks; three heart tricks; and one club trick for down 1 and an 82% board.
Partner will not find the spade switch from her side of the table if you return a heart, because she is looking at ♠AJ87. If you return a heart at trick two, Declarer can develop a trick with the ♥J109x originally, and has time to develop the diamonds and finesse in clubs. Declarer will take either seven or eight tricks, depending on how he plays the clubs: four diamonds; one heart; one spade; one or two clubs.