And at the heart of it is a simple LC tank circuit driven by an SCR. There's a coil in series with the line, and a capacitor to ground, SCR fires on, cap charges through coil, when the peak voltage on the cap is higher than the line potential, current eventually tries to reverse, SCR switches off, cap discharges (through a transformer), cycle repeats.
Well, sometimes the capacitor is a synchronous condensor and looks more like banks of motors, and the SCR is really fifty-zillion littler SCR's living in Thyristor Towers apartments there, the coil appears to have been designed by Oudin on Mt Olympus... but it comes down to a basic tank circuit and a switch.