Hand #15 at the San Diego Sectional (Open Pairs at 1:00 PM) on August 19, 2018 was interesting. Your hand is ♠Q7 ♥J10643 ♦AJ82 ♣QJ. On your right, the opening is 1♠. You pass. LHO bids 1NT (forcing one round). Partner passes. RHO bids 2♣ (convenient minor—could be as few as three cards). You pass. LHO takes a “false preference” back to 2♠ [should have a doubleton spade and minimal hand] and everyone passes.
You make a normal lead of the ♥J [hoping it will be a “tap” suit** as partner is marked with four spades on this auction] and see this Dummy: ♠109 ♥AQ85 ♦Q7543 ♣74. Declarer thinks a bit and calls for the ♥Q. Partner plays the King as Declarer plays the ♥2. Now partner plays the ♦6 and Declarer plays the ♦9. What gives?!
Trust your partner! He is on your side. Partner would not set up Dummy's longest suit, so that must be a singleton diamond. You take the Ace and return the ♦2 (suit preference for clubs—lowest suit) as the ♣Q is the only ghost of an entry that you have. Partner duly ruffs the diamond with the ♠2. Declarer thinks a little, and drops the ♦K, creating a Dummy entry with the ♦Q (maybe). Partner returns the ♣10. Declarer takes the Ace as you drop the ♣Q, guaranteeing the Jack [or a singleton].
Declarer plays the ♥9 (you duck, having no intention of setting that the ♥8 up in Dummy) and partner will be short in hearts as well. Declarer takes the ♥A and partner plays the ♥7, so you know Declarer and partner both started with only two hearts. You now know Declarer's exact pattern: five spades, two hearts, three diamonds, and three clubs.
Declarer runs the ♠10 to your Queen. You give partner one more diamond ruff and partner returns a low club to your Jack which wins. This means that partner has the ♣K. If you return a heart or a diamond, Declarer will be able to ruff the heart and then ruff a losing club on Dummy. If you play your fourth diamond, Declarer could play the ♦Q, and if partner does not ruff, throw away a losing club. So, you return your last spade to partner's King and Declarer's Ace. Declarer pulls partner's last trump, but has to lose a club at trick 13.
So, Declarer is down the dreaded two vulnerable tricks for -200.
**”Tap” suit refers to a suit with which you can force Declarer to ruff in the long trump hand until he eventually loses control of the hand because one of the defenders ends up with more trumps than Declarer.