ok its easier to break this down into subsystems of the whole system.
first is the foot pedal, on mine it is a on/off switch. when it is off the leads are dead, when its on the leads are hot. this is controlled by a simple foot pedel that is is hooked to an industrial contactor( a really big, high power relay). the pedal closes the 'contactor' and the ground wire is then connected to the machine. at the same time a smaller automotive relay is triggered by the contactor and closes a circuit that tells the high frequency arc starter box to start.
the high freq box starts the arc without touching the base metals, you already knew that, but it also has a gas valve and timer that is triggered by the contactor(foot pedal). so whenever the pedal is pushed the contactor first closes, then it triggers the relay, which triggers the high freq and gas flow. the welder is always on and the current is at a set ammount(differs according to what im welding.
its a pretty affordable way to do it, you have high freq, gas control, and non-live leads.