I will make a long story short. Honda EU-series generators are really inverters. An alternator provides power to an inverter. Output frequency is not dependent on engine speed so these units will speed up and slow down, Unlike regular generators where the engine runs at a constant speed.
Since the inverters are rather sofisicated electronic devices they have the ability to "Sense" if they are already powered when they start.. And if they are they "Sync" up with the existing power source. Just exactly like "Solar" inverters do... (They sync with the mains power.
Now... Though I likely could understand it if I did the research as to how they do it.. I have not done that.. The only thing you need to know is that the actually power source at the 120 volt level is an INVERTER, not the "Generator" part which is a DC device. This also explains the high cost of those units.
Oh, because the output device is an inverter, instead of a coil of wire excited by a rotating magnetic field.. The output is a nice clean SINE wave,, They are True Sine Wave inverters. Very good, Very kind to your electronics.
(most "Contractor" generators the output waveform is downright scary).
