I thought EASA regs stated helicopter flight wast "Engine Start to Rotor Stop" and that a "helicopter must have a rated pilot on board at all times during flight".
Never understood how this worked with solo students though. Also I've noticed that a fully rated pilot will turn down the honor of ground running an aircraft if his LPC on type has expired stating that it is not legal - so how do us lowly mechs get away with it????
On the subject of engineers doing ground runs, I do know a guy who spun an R44 on the pad when he opened the throttle to quickly. Don't think he'll do it again though.