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4+3=7
This deal is from an old International Bridge Press Association Bulletin. At favorable vulnerability, South held:
♠— ♥K Q J 8 7 3 ♦A 9 5 ♣10 7 6 2
Any reasonable method of hand evaluation would lead to an opening bid of 1♥. West preempts with 2♠ and partner bids 3♠ to show a game-forcing heart raise. With six beautiful trumps and two first-round controls, South should make a control bid, 4♦. North now jumps to 5♥. What’s that?
When the opponents have bid a suit, a jump to five of our major should invite slam if partner can control their suit. With first-round control, opener tries 5♠ and responder jumps to 7♥. A trump is led and you anxiously await the dummy:
The only possible loser in your hand is the fourth club. If clubs are 3–3 (or the jack falls), you have 13 easy tricks. If trumps are 2–2, you can ruff the fourth club in dummy. Is there anything better?
You could draw two rounds of trump and if they are 3–1, still try the clubs and hope that if an opponent has club length, he is the one with three trumps.
There is something even better. Veteran problem solvers will recognize the chance to reverse the dummy. If you can ruff all four spades in your hand, you won’t have to worry about the long club. After winning the trump lead in dummy (East follows), you ruff a spade. You cross in trump (RHO throws a club) and ruff a second spade. You can cross in diamonds and ruff a third spade. You can cross in clubs and ruff the fourth spade (RHO throwing a diamond). You have ruffed four spades in hand and drawn two rounds of trump. You are out of trumps, but West remains with one.
How do you cross back to dummy to draw it?
Since you have only six diamonds, but seven clubs, the odds favor crossing in the shorter suit. This works, as you can see from the full deal:
Drawing trump and hoping for the clubs to come in would have failed. Ruffing all four spades in your hand was the correct line of play.
A title of “Dummy Reversal” would have been too obvious; I hope that 4 + 3 (four trumps in hand and three in dummy) didn’t give away the show.