This deal was played online (during Covid-19 lockdown) in 2020. Several East players found the defense to be diagnosable. Can you?
With both sides vulnerable, East held:
It won't matter whether it is matchpoints or IMPs. The opponents reach 3NT via this auction: 1NT-2-2-2NT-3NT.
Your partner leads the 3 (standard) and dummy plays low.
The Rule of 11 (11-3=8) tells you there are 8 cards higher than the 3 in the other 3 hands (dummy, you and declarer). You can see 5 of them. That means declarer has 3 cards higher than the 3. That doesn't exactly help, but let's say you play third-hand high, and your K wins the trick (declarer plays the 4).
Who has the A? It must be partner. If declarer had AQx, he would have won the trick. With A??, he would also have won (why would he let your king win and allow you to then play another diamond to partner's queen?).
So, let's assign partner 4 HCP for the A. You have 12 HCP and dummy has 10. Subtracting from 40 leaves only 14 for the declarer who opened a 15-17 notrump. Likely he (as players often do these days) upgraded--quite possibly for a 5-card suit.
Have you figured out your play? Did you find a low club switch? It is the correct play, but not easy to find.
Here is the thinking. If you return a diamond, partner gets the A, but can never get more diamond tricks. In addition, you'll get your two aces, but that is all for the defense. Declarer has to be able to take the rest (with all the other HCP and with the hearts sure to provide 3 tricks).
Why does playing a club give you a 5th trick? No guarantee, but it is your only chance. No other suit can provide a 5th trick. If declarer has K10 and plays the 10, so be it. Still, you won't have cost an overtrick (your side can always cash out to hold it to 3). But, if declarer doesn't have the 10, you could be in business. Perhaps a sneaky J has merit (maybe declarer had K9x and will face a guess), but on the Real Deal, any club switch (other than the ace) leads to down 1. This was the Real Deal:
Several experts found the club switch. Declarer's king won. When East got in with the A, he played a diamond to partner and another club set the contract! Well done!