My experience in the AS350 is that it normally happens with full rpm, and picking it up to a hover solves the problem. Once, however, I got it after I reduced the throttle (engine lever) to idle, and it got very bad very quickly. The first thing I did was to pull the throttle completely off, and then pull the rotor brake. It finally stopped shaking after turning about 45 degrees on the pad, and observers said the heels and toes of the skids were coming a foot off the ground. I've had them try to start during runup, and immediately retarding the throttle solved it. In the case above, all maintenance could find was some wear in the spherical bearings.