When I worked as a contractor at Fox Tanker Base in the 1980's, we watched L.A. County Fire Department helicopters perform autorotation training using the eastern parking ramp at the tanker base. This ramp is used to park aircraft overnighting, being maintained, or out of service during major fires. Normally a couple of air tankers are positioned at Fox, but during major fires I have counted as many as 15 aircraft at the base. So the ramp is pretty large.
During autorotation the helicopters engine is disengaged from the main roter system. During an engine failure, this disengagement has to be performed quickly before the roters stop turning. Simultaneously the helicopter needs to be descending so that rotor blades are driven by an upward flow of air through the rotor blades. This is opposite to the normal operation of a helicopter, where air flows downward through the roter blades.
Autorotation is an emergency maneuver that helicopter pilots us to save himself and his passengers. Much as a fighter pilot uses the ejection seat that they have.
The title link goes to Wikipedia's article on autorotation.