Here's a little more explination on the vortex ring state. Its more theoretical than anything but I found it wicked cool
He continues on the next page with:
"For the pilot trying to fly under these conditions, the vortex-ring state produces some interesting effects, which we'll get into a little later.
Turning the fan up full blast so that the flow is greater than the rotor-induced velocity makes the net flow upward
through the rotor. The rotor is now slowing the tunnel flow a little and is actually extracting energy from the passing wind, Naturally this condition is known as the windmill-brake state."
Windmill-brake state is what happens during autorotation. :P
He also goes on to explain a bit more about the vortex ring creating a bubble of circulating air. It doesn't stall the rotor, it's just sucking its own downwash into the rotor, so the incoming air has a velocity, and the rotor has to do more work to keep the same amount of thrust. The bubble can burst also usually on one side first, then the next, causing the rotor to behave erratically and flap and feather, causing tilting and other weirdness.
You can actually SEE a vortex ring state with the smoke from your exhaust as your descending slowly o a zero wind day. It's pretty cool, it builds up more and more and more smoke circling through the rotor, out, up, and back down through till you have to add collective or risk hitting the ground! :P
Neat huh? It's a spectacular book, whether or not you have a background in aerodynamics (I do).
(Source: Title: Rotor & Wing International, Helicopter Aerodynamics Author: R. W. Prouty Publisher: Phillips Publishing 1985)
Just thought I'd chime in with a bit of theory :P