The importance of signaling

This is a hand from last night, as best I remember it -- let me know if I'm misremembering any important details.

North	East	South	West
	Pass	1	Pass
1	Pass	2NT	Pass
3NT	Pass	Pass	Pass

Lead:  4

North

 x x x

 K Q J 10 x

 K

 J 9 x x

West

 A x x

 x x x

 A 7

 Q 10 x x x

East

 Q x x x

 x x x x

 x x x x

 x

South

 K x x

 A

 Q J x x x x

 A K x

Sitting East, you might feel you don't have much of a role to play in the defense of the hand -- try to duck a spade finesse maybe, and return partner's suit.

In fact, the card you play on the first trick can have a dramatic impact. Declarer has an entry problem: if the defense times its cards right, he can never get to all those juicy hearts. On the second trick, he tries to get to dummy by leading his low club. West would naturally play second-hand low on this trick -- which guarantees 9 tricks for the declarer -- but if West realizes that South doesn't have any more hearts, West can go up with the queen to block the entry.

The trick is for East to signal high-low on the first heart trick, to indicate an even number of hearts. West knows that if South had three or more hearts, he probably would have supported North's bid; so he can deduce that the even number of hearts that East has = 4, leaving South with a singleton ace and no way to reach dummy's winners.