This hand comes from recent play on OKBridge. IMPS, North-South vulnerable, South dealer.
North
S: K5
H: JT52
D: 64
C: AQ964
West East
S: 62 S: 8743
H: Q4 H: K9832
D: KQT9 D: J73
C: JT532 C: 8
South
S: AQJT9
H: A6
D: A852
C: K7
North East South West
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1S 2NT At favorable vulnerability, West
3C pass 3D pass made an aggressive "unusual 2NT" bid,
3S pass 4NT pass showing a weak hand with both minor
5D pass 5NT pass suits. North-South now suffered a
6D pass 6S pass bidding misunderstanding which caused
pass pass them to bypass the usual contract of
3NT. Since West has claimed possession
of a club suit, there is little point in North's mentioning his clubs.
Consequently, many experts play that North's 3C bid shows spade support
and "limit raise" values -- about 10-12 points. South thought that
North had this hand type and consequently pushed to the spade slam.
West led the diamond king, won by South with the ace. At this point,
South could take 12 tricks as the cards lie if he were allowed to ruff two
diamonds on the board. However, once South loses a diamond, the defense
is likely to lead spades, holding South to one diamond ruff. South did
not follow this line at the table -- and for good reason. South expects
West to have five diamonds for his unusual 2NT overcall. In this case,
East will have only two diamonds, and thus East will overruff dummy to
defeat the contract.
South chose to pursue an alternate line: a "squeeze" against West. In squeeze play, the objective is to compel the defense to make discards that they don't want to make. To effect a squeeze, declarer needs to lose a trick early in the play -- a process known as "rectifying the count". Consequently, at trick two South leads the diamond two from hand; he has to lose this trick eventually, and for tactical reasons he has chosen to lose it now.
West plays the nine of diamonds, trying to win the trick as cheaply as possible. East now faces a decision: he can let partner win the trick, or he can overtake with the jack. Since it is often advantageous for the defense to lead through declarer instead of up to declarer, East decided to win this trick and play a spade. South wins in hand, ruffs a diamond with the spade king, and plays a low heart to his ace. (Note that South ruffs with the king because he expects East to be able to overruff the five; also, since South's spades are AQJT9, ruffing with the king doesn't hurt.)
South now proceeds to draw trump. After three rounds have been drawn, we arrive at the following position:
North S: H: JT D: C: AQ96 West East S: S: 8 H: Q H: K983 D: Q D: C: JT53 C: 8 South S: AQ H: 6 D: 8 C: K7West is in serious trouble at this point. If South holds the club king, then discarding a club would allow South to score four club tricks. On the other hand, if South holds the heart king, then discarding the heart queen will allow South to take two heart tricks. The play of the hand to this point, however, indicates that South is more likely to hold the club king than the heart king. (South retained dummy's clubs instead of dummy's hearts.) Thus, when South plays the spade ace to draw East's last trump, West's best discard is the heart queen. Dummy discards a heart.
But West's troubles continue when South now plays the spade queen -- the "squeeze card". West's only hope is that East holds the diamond eight. In that case, West could discard the diamond queen and retain four clubs. South would then play the king of clubs followed by a low club, which West would cover with the ten; South could cash dummy's ace-queen of clubs, but on trick 13 West's club jack would beat dummy's club nine.
Therefore, West discards the diamond queen. But now South takes the last four tricks with the diamond eight and the ace-king-queen of clubs. Six spades, bid and made.