Trump Echo

A defensive count signaling agreement when declarer plays trump, typically playing the natural low-high with 2 (or 4) trump but playing middle-low (if middle is an affordable card) when holding 3 trump.  This carding can be helpful to partner, indicating an opportunity to ruff a side suit when their side can regain lead before Declarer exhausts all the remaining trumps.  See Details
 

Trump Suit Preference

An alternative use of carding when declarer begins playing trump, allowing defenders to show suit preference.   When the defender can afford to play either of two trump cards held, first discarding the higher card as part of a high-low echo shows interest in the higher ranking suit while playing a low-high echo indicates a desire to play the lower ranking suit.  See Details