At matchpoints, you hold ♠K93 ♥Q76 ♦ J632 ♣Q98 and partner opens a weak NT (12-14) in first seat. RHO overcalls a natural 2C and it goes all pass.
Ugly lead, heh. But you settle for the ♥6 and see
♠A864
♥10852
♦K107
♣65
The first trick goes ♥6, 2, K, A. Now declarer plays ♣A, then the ♠J to the A and a club to the J, which you win.
Already we can make some inferences - why would declarer overtake the J of spades if he also had the Q? So the Queen is likely in partner's hand. And clearly the ♣K is in declarer's hand. But partner has 12-14, so where are her points? 5 of them are the ♥K and ♠Q, so she must have a bucketload of points in diamonds - the Ace and Queen (since you can see everything else). But that's only 11 points, so she must also hold the ♥J.
Armed with this information, consider what happens if you play diamonds. Sure, it looks good to play a diamond through the K to partner's known AQ - but this suit should be ringing "frozen suit" alarm bells in your head. If you do lead diamonds, declarer holding 9xx will score a trick in the suit (eg J-K-A-small; then the 10-9 team up against the Q). So best stay away from diamonds - they're not going anywhere anyway. Play the SK. Declarer ruffs, draws your trump (partner throwing a diamond) and now you know (more or less) the entire hand:
The position is now
Knowing is only half the battle though. Not playing diamonds was half the remainder - the rest is figuring out the correct defence when declarer plays a small heart. You know partner can win, but what happens? She'll play a spade back, then another heart comes which you win, and you're forced to open up diamonds. So you must win the ♥Q NOW. Play a spade, and when partner gets in with the ♥J she'll still have a spade to exit with. Declarer is forced to play diamonds himself, and partner now scores the diamond AQ and her last spade - one down and a well-earned top.
ahydra