Both Overt and oldbeefer have it right.
In an engine failure from 100 or 200 feet, you might call that an "autorotation", but it is not. Until the ROD goes very big so that the flow pushes up through the rotor [when the rate of descent is perhaps 1 1/2 times the downwash velocity (1000 fpm for a robie)] the rotor is in powered flight, powered by the rotor energy due to the inertia.
The point I am trying to make is that the rotor does not care if the engine is running, that is not the way to tell if an autorotation is taking place. In a powered, engine off descent, it is possible to get into VRS at the point in the descent where the downflow is just turning into upflow, as Overt says.