1054 AK5 83 KQJ74
KJ3 32 QJ102 A1095

You lead the Q followed by the 3, partner's 4 and declarer wins the A.
Declarer now plays the 2. Your play?

Partner's 4 signal marks delcarer with the K so there is no future in diamonds.

Looks like the only way to defeat this hand is if partner has the A. So win the A and switch to the 3.

	1054 AK5 83 KQJ74
KJ3 32 QJ102 A1095		A876 64 9764 863
	Q92 QJ10987 AK5 2

If you ducked, delcarer with his singleton club makes his contract. Now let's look at another example.

Contract 4

	A85 KQ76 A54 543
2 AJ985 KJ3 J1092

You lead the J which goes to the 3 by dummy , the 7 by pard, and won with the Q by declarer.

Declarer no leads the 2. Do you win the A and if so, what do you return?

Pard does not have the A or he would have played it at trick 1 (you may have led from KJ10x).
You don't have any quick tricks in diamonds.

 

	A85 KQ76 A54 543
2 AJ985 KJ3 J1092		QJ9 1043 10986 K76
	K107643 2 Q72 AQ8

On this hand you don't have any possible quick tricks to set the contract or even set up before declarer can pitch on the good hearts. So DUCK! DUCK! DUCK!

By ducking you only give the declarer 1 spade trick when he has a singleton instead of 2 tricks. (Declarer has to lose 2diamonds, a club, and a trump)
He will fail and not make his contract whereas if you hopped up with the A the contract is cold.(Declarer will now discard his losing club and a losing diamond)