Some TRX (or CB) are not equipped with a "PTT Remote" type RCA / CINCH jack connector transmission control socket.

This socket sends grounding information (so as to switch a relay) in TX mode.

It is used to change state of an HF accessory connected to the coaxial line (without HF detection).

Sample: switch a linear amplifier to transmission, switch an antenna preamplifier or a QRM-Eliminator to "transmission mode", in order to avoid "burning" them.

The principle is very simple: it is a simple HF detector which uses a transistor as a switch by grounding the hot point of the "PTT Remote" output.