The document said it generates electricity during towing when the front wheels are on the ground, not the rear wheels. Interestingly it says it will do this even when the high voltage system is turned off, which I assume they are referring to pressing and holding the power button for three seconds which emergency responders do to shut down the HV system and also deactivate the airbags. As to what the risk is I don't know since it seems unlikely anyone will be touching the car while it is being towed. Typical electronic devices store energy in capacitors and such for a while so there is a slight shock hazard even after you unplug the device, but I'm not sure how much energy could be stored in the system while towing that would make it dangerous once stopped.