This hand is from the 4th World Junior Bridge Team Championship which was held in Denmark from August 3 to August 12, 1993. The final standings had Germany in first place. Norway finished one half of one point behind to come in second, and USA defeated Denmark to finish third. The standard of play was excellent and this hand is reflective of what was going on during this competition.
The opening bid by East was a modern weak two bid. The subsequent bidding was natural. It should be noted, however, that this was the only North-South pair in the room to reach game, which made the defense all the more critical.
West started naturally enough with the jack of hearts. East won the king and the ace and continued with the seven, ruffed by West for the defensive book. The fate of this contract now depends on West's play leading to trick four. His choices are a trump, a small diamond, or the king of clubs. I think it is safe to say that the majority of players would select the king of clubs. After all, you only need one more trick to defeat the contract, and if the king is captured by the ace, the queen will be the high outstanding club. If, however, you consider the end position very carefully, you will see that the only lead that defeats the contract is a diamond.
If either a club or a trump is led, declarer will take five spade tricks and then play the queen of hearts. West will be squeezed in the minor suits and will have to bear down to the king-jack of diamonds in order to keep a control in the club suit. Declarer will now lead his singleton diamond towards the dummy, covering any card that West plays. When the diamonds fall favorably, the contract comes home.
West was able to amaze the audience, however, by leading a small diamond. Declarer won two diamond tricks, discarding a losing club, but was now unable to execute any squeeze and had to concede a club trick at the end for down one.
This appears to be a difficult position to analyze. In reality, West has a lot of information after the first three tricks. There are only three unknown cards in declarers hand. South is known to have five spades (at least), four hearts to the queen (North would have played the queen if he held it), and the ace of clubs (if North had this card he would not have opened two hearts). If South has no diamonds it won't matter what West does, the hand can't be made unless he also holds seven spades. If he has two or three diamonds it also won't matter, the hand can't be set. But if South has one diamond (and two or three clubs), a diamond must be led to break up the possibility of a squeeze.
If you don't see it, take a deck of cards and lay out the hand. Go through the play after each of the three leads at trick four. Yes, the king-jack of diamonds must be in the West hand for this to work, but if that is the only possibility, then you must assume declarer will play for that holding.