I guess thats the punch line before the joke then
Being type specific could be a red herring, but more likely to narrow down research to single rotor (as opposed to Chinook for example) and it may be that the Lynx semi-rigid rotor head could feature in your answer.
Probably a duff steer, but mention anyway in case you are expected to have some lateral thinking:
Air Test: - ‘over-pitching’ is often misinterpreted as vortex ring. This is where the pilot rapidly increases collective considerably and the engine cannot produce enough power to overcome the large, swift increase in drag on the rotor system. The result is that the rotor system quickly slows down and loses efficiency causing the helicopter instantly to sink. Again, this is not vortex ring.
Not sure if this would necesarily induce vibration and loss of cyclic control, but it seems quite resonable to me, anybody else?