Ionagh & Sheepguts are spot on.
The last airborne HF system I worked on used to interlock the HF Transmitter with the Aerial Tuning unit: on keying the PTT the transmitter would only radiate a couple of Watts while the ATU signalled "tuning". ( The ATU needed some RF to sense the dip in reflected RF as the aerial became matched).
Once tuning was complete the ATU sent another signal to the Transmitter, stepping it up to full power.
The whole process was quite quick, (<1sec) as each time you changed frequency, the ATU would perform a coarse tune using preset aerial data. The tune sequence on PTT was supposed to just tweak the final setting (but could force a complete coarse tune if the reflected RF was too high.
regards,
KM-H