dkatwa
Please excuse the smart a** remarks which you can always expect on this site.
If you were to stop the main rotors from turning, the helicopter would drop like a rock. All certificated helicopters are designed to autorotate in the event of an engine failure. The same applies in a tail rotor failure when the pilot initiates an autorotation. Rather than try and explain it myself, you can get a pretty good explanation here:
http://en.wikipedia.org/wiki/Autorotation_(helicopter)