These suit preference signals are very similar to the concept of Revolving Discards developed by Mr. J. Attwood. This defensive discarding method was devised by Mr. Hy Lavinthal in 1934, and has had a greater effect on defensive play than any other development in bridge history and ranks with the distributional echo and the High-Low count signal.
These suit preference signals are also known and designated as the William McKenney Signals (England) and also as the B. Jay Becker Signals (United States).
A suit preference signal is an important tool for the defenders and it can keep the line of communication open. However, suit preference signals are often abused and overused. It is important to remember that showing attitude and length through signals are assigned higher priority. Therefore, it is important to remember that a suit preference signal is an unusual play of unmistakable significance. A suit preference signal is asking partner to lead a specific suit once the partner has the opportunity. Suit preference signals can be used when discarding, when leading as well as when following suit.
The contract is 3 No Trumps. The declarer is South. West leads the 3 of Hearts. South plays the 6 of Hearts from the dummy, and East plays the Queen of Hearts in order to keep a perhaps crucial entry so that East can lead through South at the correct play.
South then leads the Queen and then the Jack of Diamonds on the second and third trick. This is followed by the last small 3 of Diamonds by South. West has held up taking the Ace of Diamonds until the third Diamond play. East is forced to discard a card on the third Diamond trick.
A standard suit preference signal would be to discard a card of that rank which should then be led. If East wanted a Spade return, then East would discard a Spade, etc.
But East sees a chance to defeat the contract, but only if West, on the fifth trick, after gaining the lead, plays a Heart. Under the circumstances above, it would be disadvantageous if East had to discard a Heart, asking West to lead a Heart. Using the Lavinthal Suit Preference Signal, East must discard the 4 of Clubs. This means that East does not want a Club lead on the fifth trick, but rather the lead of the lower-ranking suit of the two remaining suits, Spades and Hearts, and therefore Hearts.
As the cards lie with East, East is able to discard a Club without having to endanger the loss of a possible winner by unguarding the suit. In the case that East had only a 4-card Club suit, and therefore unable to discard a Club to signify to his partner that a Heart lead is wanted, then East must discard the 10 of Spades, which signifies that East wishes the lead of the higher-ranking suit of the two remaining suits, Hearts and Clubs, and therefore Hearts.
This is the first method of showing which suit is preferred, using the Lavinthal Suit Preference Signals.
A second method of showing which suit is preferred, using the Lavinthal Suit Preference Signals or Revolving Discards, developed by Mr. J. Attwood, is to treat all four suits as a circle: Clubs-Diamonds-Hearts-Spades-Clubs-Diamonds-etc. When the partnership agreement includes this method, then the discard of a low card in a suit calls for the lead of the next lower-ranking suit of the two remaining suits. The discard of a high card asks for the lead of the next higher-ranking suit of the two remaining suits.
In the example above, if East discards the 10 of Clubs, East is asking for the lead of the next higher-ranking suit, Hearts. If the holding of East does not allow for a safe discard in Clubs, then the 3 of Spades, a low card, asks for the lower-ranking suit, Hearts.