The Foster Echo is an antiquated defensive signal used by third hand against a no-trump contract. When dummy covers partner's opening lead and third hand does not have a higher card to play, a Foster Echo can be employed. It's the play of third hand's second-highest card in the suit, and is meant to unblock the suit while simultaneously providing a count signal.
From a small tripleton, third hand would play the middle card (i.e. the second-highest card), followed by the highest card and then the lowest. Playing low-high shows an odd number of cards, a la MUD
From a four-card holding, third hand would play his second-highest card, followed by the third-highest card. Playing high-low shows an even number of cards, as in standard count signals.
The Foster Echo is named for its inventor, the late American card player and writer Robert Frederick Foster.