This deal was played (and reported to me) by David Berkowitz in the finals of the 2017 Blue Ribbon Pairs. David was declarer, but I ask you to take the West hand on defense:

1086
KJ8
KJ10
J1065.

At favorable vulnerability, your partner deals and opens 4. This is passed around to your LHO who balances with a double. RHO takes it out to 4 and you try 5. LHO bids 5 and your partner doubles. Everyone passes and it is your lead.

For partner to preempt and then double, he likely has a defensive surprise. It isn't a trump stack, so probably he has a void. Trying to hit that void, you lead the J and see:

	AK97 Q AQ74 AQ94
1086 KJ8 KJ10 J1065

The Q is played from dummy. Partner doesn't ruff, but he wins the K and returns the 9 to declarer's 10 and your king. Now what?

Why did partner underlead his A? So you can give him a ruff. Did you play a club (figuring that even if he doesn't ruff this, his void must be in diamonds and you'll set the contract anyway)? That's not trusting your partner. He returned the 9, his highest--which is surely suit preference for the highest-ranking side suit. He must be void in diamonds. In fact, this was the Real Deal:

Vul:N-S Dlr: East	AK97 Q AQ74 AQ94
1086 KJ8 KJ10 J1065		52 A9765432 -- K87
	QJ43 10 986532 32

So, if you return a diamond, partner ruffs and you later get a diamond trick for down 2. But, the best part of the story is yet to come.

At the table, West indeed figured his partner was void in diamonds--but he returned the J. Why does that matter? Because declarer also knew (from the bidding and play) what was going on. He played low from dummy on the J return! How was East to know to let this win? He figured declarer had the K, so he ruffed--in effect ruffing his partner's winner. Declarer was down only one after all.

West should return the K! Now, his partner can't go wrong and the contract is down 2. You see something new every day.