The short keying is to activate the ATU as already explained.
The ATU does not really 'tune' the antenna, it is a matching circuit between the transmitter and the antenna.
If the antenna is not matched (in impedance) to the transmitter then the transmitter will not produce anything like full power and the RF radiated will be quite small.
Your HF antenna has fixed dimensions which means that it has a different impedance at different frequencies.
Therefore to get the best radiated signal the ATU will be used to impedance match the antenna to transmitter each time a new frequency is selected.