The purpose of the trump echo is to provide partner with a count of the trump suit, especially for the purpose of getting a ruff. Trump echoes should be reserved for these situations only. (Declarer gleans too much information if you always use trump echoes to give count.)
As a corollary, playing low-high in the trump suit indicates an even number of trumps. These agreements are part of standard carding, and should not be considered special carding.
Assuming that your side has agreed to lead the king from ace-king, then partner must be missing the
At trick three, declarer plays dummy's
This situation calls for a trump echo. You hold three trumps and are interested in a ruff, so you should play the
Declarer now leads a low spade from dummy. You complete your trump echo by playing the
The full deal:
Partner should have figured out the heart situation already, because declarer must hold the
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