Partner has led what is clearly a singleton. You win your ace and lead the suit back for partner to trump. You hope desperately that he will guess correctly which suit to return.

The Suit Preference Signal is used to avoid guesswork in precisely these situations. The crux is: when you are leading a suit for partner to trump, a high spot card lead asks partner to return the higher ranking of the other suits; a low spot card lead asks for the lower ranking suit.  

 West sensibly chickened out of bidding 4♠ as he was vulnerable. It gave his partnership the chance to use the Suit Preference signal. Against 4♥ he led ♣2 - an obvious singleton from East’s perspective. East won ♣A and wanted West to underlead his ♠A - even though dummy only held a singleton. So he returned ♣9 - asking partner to return the higher ranking of the other suits (♠s and ♦s). West trumped and obediently led ♠2. East won ♠K and led another ♣. West trumped a second time and the defence had taken the first four tricks.

ANDREW’S TIP: When leading a suit for partner to trump, a high card asks for a return of the higher ranking suit, a low card asks for the lower raking suit.