Hands are from San Diego Unit Game, November 4, 2018.
North (Dummy): ♠A ♥A75 ♦KJ843 ♣KJ74
East: ♠84 ♥K982 ♦A109762 ♣10
South opened 1♠. North responded 2♦. South rebid 2♠ and North rebid 3♣. South persisted with 3♠ and North raised to 4♠. Partner made the normal fourth-best lead in hearts—the unbid suit. Declarer took the ♥A as you signaled encouragement in hearts, and cashed the ♠A. She then played a low diamond from Dummy.
There are only two diamonds outstanding and one of them is the ♦Q. Do you hop with your Ace? The defensive inference here is that partner would have probably led a diamond if she had a singleton in the suit, so partner either has the ♦Qx or partner has a void. Either way no need to play your ♦A.
Declarer plays the ♦Q and partner ruffs. Partner plays a heart to your ♥K and you cash the ♦A and then play your lowest diamond (suit preference for clubs), hoping for a trump promotion. Alas, Declarer ruffs high and pulls all the trumps. It turns out you knew too much. If you had jumped up with that the ♦A, partner would have discarded. You could have given her a diamond ruff with your lowest diamonds and THEN she could cash the ♣A and give you a club ruff. Then you cash the ♥K and try for a trump promotion via another diamond play. (Double-dummy, you lead a heart to her ♥Q and get a second club ruff. How do you know partner has the ♥Q instead of the ♥J? Again, defensive inference. A decent Declarer would DUCK the original heart lead if she had ♥Q doubleton, rather than flying with the ♥A, so Declarer should have ♥J, NOT the ♥Q.)
East (Dummy): ♠J3 ♥AK103 ♦952 ♣AK72
North: ♠86 ♥QJ752 ♦QJ104 ♣Q5
Declarer is playing 4♠ and leads a low heart (or perhaps the ♥9). As a defender it is INCORRECT to split your honors. With the ♥10 in Dummy, you give the whole show away if you play the ♥Q or ♥J. Declarer may or may not be intending to finesse you for both heart honors. If you split your honors, Declarer will DEFINITELY play you for both heart honors and will take four heart tricks. Remember, Declarer canNOT see your cards. (If she can see them, hold your hand back further!)
East (Dummy): ♠AJ106 ♥J5 ♦10872 ♣852
South: ♠9 ♥AK10932 ♦QJ9 ♣KQ6
Partner (North) passed as did East. You opened 1♥ and West overcalled 1♠. Partner passed and East raised to 2♠. You rebid 3♥ and West took the push to 3♠, after which everyone passed.
Partner led the ♥Q, which held as Declarer played the ♥6, and continued with the ♥7, which you won, with Declarer playing the ♥8. What should you do next?
Our defender played the ♠9 which was the worst of four possible choices. Leading a singleton trump can kill (“pickle”) your partner's ♠Qxx(x) (or ♠Jxxx with a different Dummy). You have two perfectly safe leads: (1) a third round of hearts, letting Declarer ruff in Dummy. Partner definitely had a doubleton heart—he would have led LOW from ♥Q74 of hearts. (2) the ♦Q. Even if Declarer has the ♦A and the ♦K, leading diamonds ONCE from your hand does no damage, because you retain the ♦J9 over Dummy's the ♦10. Even leading the ♣K is not a terrible lead. If Declarer has ♣AJx, she is already likely to play you for both ♣K and ♣Q anyhow based on the bidding.
In defense of poor South, as declarer I was planning on playing him for a singleton spade based on rebidding his six card suit at the three level opposite a passed partner, and finessing his partner for ♠Qxx, but several pairs did not make that choice in the spade suit. Six pairs went down in 3♠.
Once I got the favorable spade shift, I played super-cautiously to just make 3♠, but you can make 4♠. If you play South for ♦QJx in diamonds, you cash ♦A and the ♦K before playing all the trumps. Then, enter Dummy with a trump and ruff third round of diamonds. Return to Dummy, pulling remaining trump, and discard a club loser on the good the ♦10. Pairs who misguessed the spades still had a chance to make 3♠ if defenders took two high hearts and shifted to the ♣K (or the ♦Q). Declarer can cash two top diamonds, and play two rounds of spades, ending on Dummy. The ♠Q does NOT fall, but you ruff a diamond and ♦Q falls. Then ruff your third heart on Dummy and discard a club loser on the ♦10.
If defenders starts with three rounds of hearts (removing one Dummy entry), you don't have the entries to set up that the ♦10 and get back to it IF you misguess the spades.