Auto rotation is when you reduce the pitch of the blades down to zero.
Also allowing the blades turn faster then the engine speed is called auto rotation.
Gyro copters are all auto rotation machines.
When a helicopter loses an engine the pilot can (on most small choppers) let the rotors free wheel and try to land the craft with the rotors free wheeling.
When your about 10 feet from the ground you pull back the cyclic control and the blades titling back will act like a big super drag chute and if you do every thing right you can land it with no or little damage
We usually call it a controlled crash.
Some good info here
Vegas/ http://cybercom.net/~copters/pilot/autorotation.html

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Vegas/
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