I think I remember reading or seeing a video back when I was trying nitro helis that its a really bad idea to try and hold your heli down when the blades are turning. If I remember correctly it was something about ground resonance can exponentially progress without have the freedom to buffer itself. The cure for real helis is to lift off and get out of ground effect I think was said. Same thing with having a free floating head on a run up stand to allow for that to work itself out.
Has that train of thought changed since then? If not it might be a topic to put out for the many beginners setting up their first helis.
It really depends on the number of blades and head design. If you are running a multiblade heli, then you can get ground resonance, especially if have a fully articulated rotor head (independent vertical blade flapping and lead-lag hinges), that is solved by finely tuning strut and tire pressures to damp out any oscillations. This is why most RC multiblade heads are either rigid or semi-rigid designs. We actually regularly do restrained ground runs on the full scale birds to test out components, and it is fine with proper precautions. And yes, in normal operation if the oscillation does begin to develop, lifting off the ground will immediately get you out of that situation and give a chance for the rotor oscillations to damp out.
Fundamentally, ground resonance happens because the rotor system gets out of balance. Picture you are looking down at a 3 blade head. Each blade can lead and lag pivoting about their blade bolt. In ground resonance, you end up in a situation where 2 blades are lagging and one is leading or vice-versa. In flight, this would damp out and equalize. However, feedback from the ground can cause this condition to worsen. Now, you have an out of balance rotating system that has to release of all that rotating energy and can shake apart the aircraft. If you look up videos of ground resonance on youtube, it can be pretty devasting. This doesn't happen to a teetering 2 blade heli, where the blades are linked by a feathering shaft, like our RC birds.
Where RC Helis get in trouble is one of 2 situations. Running negative pitch on the ground can get you in a near ground resonance state, and cause a lot of vibration quickly. The other issue is older FBL units having trouble dealing with feedback from the ground. The align 3G in particular really hated running on the ground, but would calm down after liftoff. Most modern FBL controllers deal with that without a hitch now.