This deal (based on one from the 2009 Team Trials) has a good defensive lesson.

Vul: N-S Dlr: South	K J 6 5 7 4 3 2 Q J A 3 2
4 3 8 5 K 10 8 6 4 3 Q 9 5		Q 10 9 10 9 6 A 9 7 J 10 8 4
	A 8 7 2 A K Q J 5 2 K 7 6

After a straightforward Stayman auction (most players show hearts before spades), South lands in 4. West happened to hit on a diamond lead.

The pair was playing 3rd and 5th best leads, so the 8 was chosen.

What should East do when he wins the A?

It would be very careless to return diamonds. Why?

I know you can see that West's K will be the second defensive trick. Declarer has to also lose a trick in each black suit for down one. Right? Not so fast.

West is not a mind reader. When you return a diamond, how is he to know he hasn't struck gold? Who is to say you don't have a doubleton diamond? How would West know not to play another diamond and hope you could overruff dummy? If he tried that, declarer would happily ruff in dummy and throw his club loser to make his contract.

West North East South       1NT Pass 2  Pass 2 Pass 4 (All Pass)

Why torture partner? You know he might go wrong if you return a diamond at trick two. A careful defender returns a passive trump at the second trick. Declarer draws trump and goes about his business. That business will produce four losers and no defensive accidents; down one.