Source: www.acblunit559.com

Smith Echo
The Smith Echo is a popular defensive signal against notrump contracts. After declarer wins his first trick, he often leads his own long suit.   When that occurs, the partner of the opening leader plays high-low to show that he likes partner’s led suit; conversely, failure to play high-low usually shows a lack of support for partner’s led suit.

Foster Echo
Against a NT contract, when 3rd hand cannot cover the dummy’s card, that hand should play the 2nd highest card.  With 4+ cards, 3rd hand plays 2nd, 3rd, and then the lowest card.  With three cards, 3rd hand plays 2nd and then lowest card.  When played in this manner, the Foster Echo shows count while retaining the highest card for later use.

Trump Echo
When declarer begins drawing trumps, a defender holding an odd number of cards plays high-low and plays low-high holding an even number.  Discarding high-low tells partner you have a third trump which may be a winner or may be useful in trumping a side-suit lead.  Of course, a player holding three trumps must be able to afford the high-low discards.