Source: “Case for the Defense”
Victor Mollo
Victor Mollo
Wikipedia:Victor Mollo (St Petersburg 17 September 1909 – London 24 September 1987) was a British contract bridge player, journalist and author. He is most famous for his “Menagerie” series of bridge books, depicting vivid caricatures of players with animal names and mannerisms through a series of exciting and entertaining deals—bridge fables of a sort. Mollo attended Cordwalles School but neglected his studies and devoted himself to bridge. As an editor in the European service of the British Broadcasting Corporation, he began to write books and articles on the game. After retirement in 1969, Mollo started to write even more extensively, and up to his death in 1987 he wrote 30 books and hundreds of articles. He was also active in developing bridge cruises, mostly in the Mediterranean. North deals.
West North East South
  1 Pass 1
Pass 3 Pass 3
Pass 4 End
Contract: 4
Q J 7 6 K Q 10 9 6 A K Q A
A 8 2 A J 3 2 8 7 6 5 4 3
West leads the 4. The K is played from dummy.

How can East make certain of defeating the contract?


By going up with the A and returning the J, the key card.

The 4 is obviously a singleton, but two aces and a ruff come to only three tricks and West cannot have a third trump.

To beat the contract it is essential that West should return a diamond at trick three and to make certain that he does, East, at trick two, leads the J, an ostentatious card, for he must have lower hearts. West will read the J as a SUIT PREFERENCE SIGNAL, calling for the return of a diamond, the higher-ranking suit. SUIT PREFERENCE SIGNAL, named after McKenney in Britain and after Lavinthal in America, were invented by the latter in 1934. They consist in playing an unnnecessarily high card to call for a higher-ranking suit. Trump are excluded. Observe the difference that a diamond return makes on the hand shown below. The defence come to six tricks: two aces, two hearts ruffs and two diamond ruffs. Three down. Should West return cluba trick three declarer cannot fail to make his contract.
Q J 7 6 K Q 10 9 6 A K Q A
4 3 4 7 6 5 4 3 2 K J 9 2 A 8 2 A J 3 2 8 7 6 5 4 3
K 10 9 5 8 7 5 J 10 9 8 J 10