It's called a "Load Genie" unloader, if I recall. Grainger.com has them.
My little 1/2 horse compressor has a single cylinder Speedaire pump, and the check valve on the tank allows the pressure on the pump side to bleed off after the pressure switch shuts it down, so it always starts under no load.
My Changfa powered 3.2kW generator starts it like it wasn't even there (capacitor start motor), but the generator totally goes tits up on the 3/4 horse well pump (2-wire split phase).
Older compressors usually had head unloaders (constant run controls) and instead of shutting off the motor, they opened the intake valves and unloaded the pump. My father has a 1 hp Binks compressor from the 60's that runs like that. Great compressor.
I'm not sure what compressors today (most of which are that cheap, direct drive, noisy as he!! junk) unload the pressure side of the pump after shutoff, but that could be an issue for a generator.
Chris