Two different ways to help the arc jump the gap when you start welding. Believe the capacitor method results in a little higher amperage during the initial arc ( lasting a fraction of a second). The high freq is doing the same thing but by changing the sine wave to make it easier to jump the gap.. Initially capacitors were about the only way to boost the ability to start an arc due to cost. the advancement in inverters and electronics have evened the cost among manufacturers.
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Josh
Josh