This deal was played in the U.S. 2017 Team Trials. South held:

AK3
J109843
K85
K.

With neither side vulnerable, he dealt and opened 1. LHO overcalled 1 and responder bid 2. This promises a limit-raise or better in hearts. RHO made a strong spade raise, but South eventually settled in 4. The J was led and declarer saw:

Q6 AK6 Q107432 96

AK3 J109843 K85 K

Things went poorly. East took the A and gave West a diamond ruff. Now came a club to the ace and another diamond ruff for down 1.

So, what's the point?

Let's look at the Real Deal:

Vul:None Dlr: South	Q6 AK6 Q107432 96
J9842 Q2 J J10754		1075 75 A96 AQ832
	AK3 J109843 K85 K

Without the ruffs, declarer has an easy 11 tricks. With the 2 ruffs, he was down one. The question is, after trick 2 (when West ruffed the diamond) how did he know to cross in clubs (not spades) for the second ruff?

East gave a suit preference signal--the 6. By returning his lowest diamond, he was suggesting the lowest-ranking side suit (clubs). West obeyed the signal, but declarer was the one at fault. Why?

Declarer followed to the first trick with the 5. West carefully watched the spots and observed that the 6 was the lowest (thus clubs). The 4-3-2 were in dummy and declarer had revealed the 5.

Declarer should have hidden his 5. If he plays the 8 and king to the first two tricks, West has a guess. Was the 6 a low card from 96? Or, maybe the 6 was a high card from an original holding of A65 (with declarer holding K98).

At least declarer should have made West guess. With all the things in planning to worry about, how can declarer worry about such things? How can he figure this out without thinking forever?

Maybe you have heard that declarer should signal as if he is a defender. If he wants the defender to continue a suit, he encourages. For example, LHO leads the A from AK. Declarer has Q92. Declarer plays the 9, "encouraging" (assuming standard signalling). By hiding the 2, he makes it hard for LHO to read his partner's card. West is more likely to continue. Of course, it helps if you can also do this in tempo, without thinking forever about the "falsecard."

The same thing applies for suit preference. Here, declarer wanted West to play the high suit (spades) at trick 3. Accordingly, declarer should have played high diamonds and kept the 5 hidden.