CJ - I am sure it will be covered in your conversion course but in summary, the high lifiting power of the 5 blade head, the faster servos and the lack of damping on the collective channel could lead to a PIO situation (I hate that term because it implies its the pilot's fault, but never mind!) where an input to the collective when on the ground (lets say down) results in the airframe (and hence collective pivot) moving down whilst the inertia of the pilot in his sprung seat etc means his arm gets "left behind" so that he makes an involuntary input in the up direction, then the process repeats in the other sense and you get divergent oscillation.
It only occurs when the trim release is not pressed (something to do with the firmness of coupling between the pilot and the lever) so the moral is to always use the trim release when moving the collective on the ground. In fact its possible for the pilot to make an involuntary input to the collective without pressing the trim release, eg when taxying over a bump etc, so the rule we have is to either be pressing the trim release, or to be not gripping the collective at all. So remember it can't occur in flight and it won't occur if you are pressing the collective trim release.
Eurocopter have now modified the AFCS software so that on the ground, the system is monitoring for the phenomena and can put in an anti-phase input to the collective series actuator to prevent the effect from ever materialising. Bear in mind that in the 225, you engage the AFCS before first taxy and don't disengage it until you shut down.
If you do get it (because you didn't comply with the above), you just have to let go of the collective and it will stop instantly. However that can be easier said than done!
HC