Good dummy play is an asset at bridge. For an average plus player, the options available are self-evident; failing which he gives up. But the expert has the vision and the foresight to look beyond and find the only chance to wriggle out of a tight spot and take the winning line for the contract bid.
That he succeeds most of the time is because he works hard at the table to conjure up the opponents' distribution and cards in the light of the odds of bridge and the percentage play that can best suit him to develop the tricks he needs for the contract to succeed. Here, playing the WS Power Precision, south, opened 2NT on the following hand, showing 24-25 high card points, and was raised to 3NT by north.
The opening lead was 10H. Looking at the dummy, sitting in south's chair, what are your chances for your contract of 3NT when you have only 8 sure tricks? From where can you extract the 9th trick? As you analyse the dummy, two lines and chances present themselves. One, of course, is the even spade break, giving you recourse to the 4th spade winner in dummy after you gain an entry with diamond J or Q, once the KD is knocked off. So, naturally, you try the first option by playing out AKQ of spades. But tough luck, as you can see spades do not break with west clinging on to the JS. East sheds a heart. Now what?
The club options are remote since you have to give up 2 club tricks for running the suit. By that time it would be too late. The defenders would take at least 2 clubs, a spade trick and 2 hearts, if not 3.
While the average player, shifting his attention to the diamond suit as the only hope, can think of only one distribution which could give him the chance of running dummy's diamonds, and that is to place west or east with doubleton king. But as the cards lie you can foresee that diamonds are not breaking too well for the declarer either, for the KD is a 3-carder lying with east who sits over dummy's QJ82.
It is in these tough situations that the expert stands well above the average plus player. For he has the imagination to combine all the chances for himself and come up with a neat solution, as did the American expert in a major tournament in 1999.
Taking the same premise of Kx being the ideal solution for the winning line, the declarer could visualize an additional chance for himself in case the king of diamonds was not a doubleton but a three carder. But to give him the best chance, he needed first of all to know the location of the KD. If it lay with west, and it was a doubleton, the first diamond from hand would confirm it. For with Kx, west could ill-afford to duck. In keeping with his option of discovering who possessed the KD, declarer played the 4D from hand.
When west played low, declarer's first option of the opponent's distribution holding a doubleton king of diamond was gone. Surely west either held a tripleton KD or none at all. He weighed the odds carefully. If west held KD tripleton, what were his chances? Very little, of course, unless west also held 10D. That would leave the only chance left in hitting the 8D in dummy. Of course, if east held 10D, there was no escape unless east held doubleton K10. So the 8D gave him higher odds for the winning line because it would also succeeded in case east held a tripleton KD without the 10D.
As you can see from the cards, east is in a quandary. Either way, he cannot prevent declarer from running his diamonds in dummy. If he holds up, he loses the KD altogether, being under the direct finessing gun. If he takes it, he cannot prevent the declarer from making his diamonds good in dummy. Such clear thinking of combining all chances is what makes bridge experts play the way they do to take the only chance.

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7542       J963       108      AKQ
QJ        10983     86542       AK
QJ82       1073       K65      A94
842          Q6      KJ10    A9753
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