In Whist the play to a lead of a high and then a lower card is a
signal for a lead of trumps, and is necessarily an indication of
strength in trumps.
In Bridge this convention is used to show control of the third round
of the suit led. Some players use this echo or signal to show that
they can trump the third round of the suit, while with others it
implies control of the third round either with the high card or by a
“ruff.”
This signal should not be used except on your partner’s lead of king,
as otherwise it would give information to the dealer. To use it
on the adversary’s lead would help the dealer to make successful
finesses, and to mark the distribution of the suits. Be careful when
you use this signal not to echo with an honour. If you hold Q x, J
x, 10 x, do not play the honour to the first trick; not only is your
partner apt to think you have no more of the suit, but these honours
are too valuable to throw away.