There is more to this than the legal guff, but in the UK this may be relevant:
UK ANO
Article 256.—(1) An aircraft is deemed to be in flight—(a) in the case of a piloted flying machine, from the moment when, after the embarkation of its crew for the purpose of taking off, it first moves under its own power, until the moment when it next comes to rest after landing;
[Another bit of the ANO says you have to have a pilot at the controls if you are in flight. By default then, if you are NOT in flight (as defined) anything goes].
So, if I understand it correctly, our leaders have decided that it is perfectly legal for a non-pilot to be in control of a rotors-running helicopter, so long as you do not plan to get airborne.
I recall some while ago, we were all told that we should never let a student start-up without us, following an accident on the pad.
As a piece of legislation trying (presumably) to control risks on a rational basis, it has more holes than the swiss cheese.
Am I right in thinking that an accident occurring during ground runs may not even be required to be reported as an accident, on the grounds that it did not happen during flight (as defined)? Just an engineering incident!
For me,(I think prompted by some pilot mentor in my past), if the rotors start to turn, I am flying it. Harness on, headset/radio on and ready for anything. The hazards start up as soon as the rotors turn: the rotor blades do not know what the intended purpose of the start is.