The actual Single Ended Triode control is a special gain-staged saturation that’s asymmetrical. It’s a little like PurestWarm, only not, because rather than put a soft saturation on one-half of the waveform, it offsets everything and goes into the saturation with a bias voltage. Then it subtracts a related voltage, and scales the whole thing up or down based on how much distortion you’re looking to get. (shown in the source code, of course)

That means you have a ‘second harmonic generating’ asymmetrical distortion, but with NO crossover point. Unlike PurestWarm, SET is a continuous waveshaper just like using a real triode tube single-ended, and while you can crank it up to get obvious effects, its real magic is in using just a tiny amount to warm and sweeten things.