Lesson 22
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22.1 Roundly chastise those who have risked it by telling them that when an opponent makes you a present of an unwinnable contract, you don't put yourself into the situation of losing it.
22.2 The (good) pupil will object that perhaps the declarer has a third Diamond and ducking serves no purpose: don't forget to salute such clear-sightedness but explain that here it was the only chance there was to defeat the contract.
22.3 Ask the declarers nevertheless to think what they would have done if East had had the presence of mind not to take the Jack of Diamonds (or what they did do if that was the case). They must answer that there is no more hope in Diamonds (advice 22.4) and that they must therefore seek their happiness elsewhere. Where, then? In Clubs of course: by finessing the King of Clubs, then, finding the suit split 3 - 3, South will total nine tricks: four Clubs, two Spades and two Hearts … and the Diamond! Congratulate loud and long whoever found the solution … and confirm that it doesn't work in this deal because the Clubs are split 4 - 2.
22.4 Insist on the reason for which there is no further hope in Diamonds: because the dummy is deprived of an "exterior" entry and declarer will have no more Diamonds in hand to get over to those in dummy.
22.5

Here again, a particularly alert pupil could object that ducking twice could cost you a trick if the declarer only had two Diamonds at the start. To which you can reply that:

1) it is better to offer one trick to declarer rather than three …
2) you would have to find a method for West to tell his partner how many Diamonds he has.
Perhaps this method exists, after all?!!
22.6 Add, nevertheless, for your good pupils that, forcing the declarer to "exhaust" an exterior communication cannot be bad fundamentally …