The high-frequency arc is an extremely low current arc. I have seen welders light the high-frequency only on their fingers with no harm done. As others have said, the high-frequency start is quite useful to create a very soft start with maximum control of the arc and no requirement to touch the material with the tungsten. The 2nd use is with AC current. You must have the high-frequency on continuously in order to be able to weld using AC current.
The reason the high-frequency works for these 2 uses is the same. The main arc is a low-voltage/high current arc, while the high-frequency arc is a high-voltage/low current arc. The electrical resistance of the extremely dry shield gas is high enough that the extremely low voltage of the main arc power cannot jump the gap from the tungsten to the part and establish the main arc.
The high-voltage/low current high-frequency power supply is the opposite. The high-voltage it can jump the gap. When it does, the high-frequency also ionizes the shield gas, causing it to turn into a plasma. The plasma has an extremely low electrical resistance, which the main power supply voltage can jump. Once the main arc is initiated, it maintains its own plasma to stay lit. this means that the ark can be lit at what would otherwise be an extremely low current.
With AC current, it is exactly the same, only different. As the AC voltage cycles from 0 to maximum back down to 0 to negative maximum and then back up to 0 over and over it continually goes to the 0 point. When there is 0 voltage, there is also 0 current, which means the arc shuts off. With the high-frequency arc on constantly, the high-frequency arc is providing the low electrical resistance plasma for the main current to pass through as the voltage increases from 0, thus reestablishing the main arc 120 times per second at 60 Hz.
