Just to clarify, slope soaring is akin to an autorotation as one is not using power. But in an autorotation, one must land, as he only has a limited amount of energy left in his rotor after lose of power, so he descends and land.

As in this video:

http://www.youtube.com/user/PreddyGS.../0/I-Pn4KNf-RU

Slope soaring is akin, in that does not have a power source inside the heli (or airplane for that matter), but one is using the energy in the slope's upward wind to maintain rotor speed, rather then the descending speed of the heli (as in an autorotation) to maintain the rotor speed.

In both cases the heli is effectively in an "autorotative state" but different energy sources are being used to maintain flight and control.