I was told that South bid diamonds then hearts, North bid spades, and the final contract was Six Diamonds. Only the West and North hands were shown to me and I was directed to lead [2. This went to the five, king and ace, and declarer immediately returned [3. "What do you do, and why?" "I go in with [Q, then return one for partner to ruff." "Why?" "Because declarer began life with [A J 3 - partner cannot have [J otherwise he would have played it rather than the king when I led [2 - a lead away from an honour. He wouldn't expect me to underlead an ace against a slam in this situation, and by playing the king he is denying the jack."
Apparently, what originally happened was that South, after capturing the club king with the ace, immediately returned [3. West played low, expecting East to take the trick with jack. But even when dummy's 10 won, declarer wasn't out of the woods. He still needed to ruff both a heart and a club in dummy, even after discarding dummy's two remaining clubs on his king and queen of hearts, therefore couldn't use trumps for getting to and fro.
What he did was cash the ace of hearts, then play the king of spades from dummy. East won with the ace, but with no clubs left returned a diamond, taken by South in hand. This untangled declarer's entries and he was now able to discard two clubs on the king and queen of hearts, trump a heart with +J, return to hand via a spade ruff, ruff his last club with +Q - and his last three cards were winning diamonds.Reuse content