It's pretty impressive, really, that a VCR-sized box can take any random voltage DC and convert it to AC precisely in step with whatever is already there, handling about ten or twenty times as much power through it as it could comfortably dissipate; yet "let go the rope at once if it sees nothing pulling on the other end" for the benefit of anybody working on the mains side.
I suppose it's basically an arbitrary function generator with a thwacking great class D amplifier tacked on the output, with the twist being that it determines what it's supposed to generate by sampling the existing waveform while the PWM output is off .....
I must see how mine handles a loss and reconnection of mains. (It's handled loss of DC every night and re-establishment every morning, so that's less interesting
) There are big, scary isolator switches -- black and white on the DC side, yellow and red on the AC side -- right next to it in the loft, so I could have a look one Lunchtime while it's got plenty of DC available!