I always thought it was (predominently) a function of the autorotaive section of the blades moving spanwise as a result of inflow angle changing.
With flare effect the inflow angle increases so the autorotative section moves outboard, and vice versa. Obviously the corresponding movement of the rotor drag sections combined with the autorotative section will have a dramatic effect on the rpm.
In low inertia head types such as the S-76 or EC-155, abrupt bunting can result in eye watering Nr droop.