On your last V2L thread a while ago, I suggested you could use a manual changeover/transfer switch (mains-off-generator orcar for example) or as they have done.
Not as I want to really get involved in this but, its not all as initially seems. It involves a bit of resistor trickery across two pins (wires) in the wiring of the the plug and the correct sized cable matched to what resisitor you are using, so not to overload the cable ?