A suit preference signal is the play of a specific card to tell partner you want a particular suit returned. The card does not relate to that suit, but asks for a card of a higher or lower ranked suit to be returned. Suit preference signals (for which McKenney and Lavinthal are named), are subtle but effective.
The play of an unnecessarily high card asks for higher ranked suit to be returned, and a low card asks for the lower suit.
KT842.A82.Q53.84 Q65.T76.9742.952 973.KQJ5.J6.AKQ6 AJ.943.AKT8.JT73
East opens 1NT (15 – 17, balanced), and West, holding enough points, goes straight to game, 3NT. South leads the 4♠, 4th highest of the longest and strongest. Dummy plays J♠ and partner’s Q♠ wins the trick. Partner returns 6♠, declarer plays the 7♠ Here is a chance for a suit preference signal to help partner know where your entry is, as you would like to ask for a heart, as you hold A♥
It wouldn’t matter which spade you played, as dummy must play the A♠ anyway. But if you play a high spade (K♠, or 10♠, or even 8♠) this is s suit preference signal for the higher -ranked suit to be returned, clearly a heart.
Suit preference signals are subtle, but look out for your wonderful partner trying to give you all this extra information!