[davrosthedalek]

I can how it would not switch on if line power is gone. But what if it is on already, there are not many loads on, and the mains go down? Does it switch off? How does it know?

[bluGill]

It watches the incoming AC waveform and matches that. If there is no waveform it has nothing to match and so it shuts off.

[davrosthedalek]

How do you watch the incoming waveform if you drive your own waveform on the same line? I guess you could look periodically.