Partner: ♠9 ♥KJ6 ♦AQ3 ♣AQJ874.
You: ♠K64 ♥Q532 ♦J64 ♣K65
Partner opens 1♣; RHO overcalls 1♠ and you make a negative double. LHO passes; partner jumps to 3♣; RHO passed and you bid 3NT.
The lead is the ♠5. RHO takes the ♠A and returns the ♠3. You duck and LHO takes the ♠7 and plays the ♠J which RHO overtakes with the ♠Q while you take your ♠K.
Now what? Should you knock out the ♥A, take the diamond finesse, or doe something else?
Your big clue is that RHO returned the ♠3 at trick two, suggesting only 5 cards in the suit. LHO, therefore, had four spades to the Jack, but did not make a simple raise OR a preemptive raise of partner's overcall. It is extremely likely that RHO has the ♥A and the ♦K is likely as well.
IF your inferences are correct, RHO will be SQUEEZED if you simply run all six club tricks. A savvy RHO will discard a low diamond on the third round of clubs, then a highish heart (standard signals); then another low diamond; then an echo in hearts. A not-so-savvy RHO will discard high-low in hearts (confirming possession of the ♥A); a low diamond and SQUIRM on the sixth round of clubs. In the second case, if RHO discards a spade, you know you can afford to knock out the ♥A. If RHO discards another diamond, you have the courage of your convictions that RHO started with five spades (strongly inferred); two clubs—known; three hearts—very likely based on discards, and three diamonds.
Therefore, take the ♦A, felling the now-singleton ♦K and make your contract.
Declarer ducks two rounds of spades and you look at dummy in despair as you realize you will be SQUEEZED on the sixth round of clubs. Your hand is ♠AQ1032 ♥A84 ♦K72 ♣32 So, the only HOPE of setting the contract is to look like someone who does NOT have the ♦K. You first discard should be a low diamond (standard carding); your second discard another low diamond; THEN a low-high in hearts—false carding there as well.
In actual fact, you cannot ever succeed if Declarer runs the clubs and then cashes the ♦A—IF you never discard a spades—and knocks out the ♥A if you do discard a spade. However, it is good practice to get used to baring an honor EARLY when discarding, and "without a care in the world" to try to deceive Declarer.