[pilowsky]

This hand came up today in a free matchpoint tourney.
Using standard bidding methods, my partner arrived in what turned out to be a makeable 4♠X.
Except, after winning the first trick with the ♠4, South proceeded to draw trump playing the ♠5 (♠7) ♠K to East's Ace.
East now has the opportunity to defeat the contract.
My questions are: What must East return? And, Does East have a reasonable way to find it?

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Here's the whole hand

Spoiler

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[Cyberyeti]

Small diamond to remove the diamond entry before it's useful, as declarer you need to run W and dummy out of hearts, so pard should have played a diamond at trick 2 so he could ruff one when E ducks and then when E wins the second diamond he will allow you to ruff the third heart, and now a spade to the KA endplays him.

[apollo1201]

Small D to kill dummy’s establishable suit (partner has to give you the count on that). Sometimes you’ll end up -690 instead of -590. No bid deal at IMPs if you can score +100, but +450 seemed makable at 5H. Good bid by S!

BTW with a 2-suiter that you over called 4S with on opps H, when Xed, is it better to run to the 2nd suit? Maybe it depends on who Xes, here we just discovered opener had a big hand that forfeited slam when partner passed so just values and not a trump stack behind us?

I had opps ending up in 5SX-2 vs. 4SX-1 recently.

[AL78]

apollo1201, on 2021-October-24, 04:09, said:

Small D to kill dummy’s establishable suit (partner has to give you the count on that). Sometimes you’ll end up -690 instead of -590. No bid deal at IMPs if you can score +100, but +450 seemed makable at 5H. Good bid by S!

I'd have been tempted to bid 5♥ with the West hand although there is a small chance of pushing them into a making slam, so I'd be hesitant.