First the High Voltage HF AC gets the tungsten "hot", then the flowing shielding gas gets hot and creates plasma. This allows the gas to conduct current. Just like the four foot fluorescent bulbs.
On AC it stabilizes the arc to force the AC to pass through the "0" line when it changes direction from electrode positive to electrode negative, negative to positive. The time at electrode positive provides the oxide cleaning action desired for aluminum. On squarewave ac machines the change to positive to negative, negative to positive is pretty fast and hf could be turned off. Not as smooth and arc but it can be done.

This ought to do it

http://millerwelds.com/education/TIGhandbook/

Hi-Freq. stabilizes the AC arc by ionizing the gas shield. This ionization enables the restart of the AC cycle after it passes through zero potential 120 times a second. ( in 60 Hz countries).

Hope this helps

brande