Normally, I'd call this "card reading," but there were no real "cards." This was one of the last deals played by Justin Lall, who died way too young. He played this deal online in the 2020 USBF Invitational Trials and did a great job of reading his "screen."

As South (nobody vulnerable), he held:

J10974
76
A6
AKQ7.

He saw 1 Pass 2, so he overcalled 2. Opener raised to 3, his partner raised to 3 and Justin (who could have had much less) raised himself to the spade game.

The A was led:

Vul:None Dlr: West	A852 83 QJ108 J42
KQ6 AKJ1092 3 1085		3 Q54 K97542 963
	J10974 76 A6 AKQ7

West	North	East	South
1	Pass	2	2
3	3	Pass	4
Pass	Pass	Pass

With the diamond finesse on, declarer can make the contract if he loses only 3 tricks in the majors. If West cashes two hearts, declarer's task is easy (as long as he guesses spades).

However, at trick 2, West shifted accurately to his singleton diamond. The Q won in dummy. Declarer correctly read the diamond switch as a singleton. Why?

Who would switch to a diamond away from the king in this position? Nobody. Even switching to a diamond from a bunch of small ones would make no sense. A good defender (which West was), would either cash another heart, or try clubs as the likely source of defensive tricks.

Accordingly, declarer crossed the to the A to lead the J. West covered, dummy's A winning. Now, another spade would have been fatal. West would win and guided by East's signal at trick one and on the second round of spades, would know to cross to the Q to get a diamond ruff for down one.

But, declarer saw this coming. Since West likely had only one diamond, it made it likely that he had the missing high and low spade. The solution? Lall played a club to the jack and then continued clubs. On the fourth round of clubs, West had no answer. If he discarded, a heart would go from dummy and declarer would actually make an overtrick. In fact, West trumped the fourth club, overruffed in dummy. Now, declarer had to lose only three tricks in the majors. Well played Justin!