The term autorotation refers to the rotors turning without the engine.
A scenario in Arma where this could be used would be whenever you are in a helicopter, flying, and run out of fuel or have an engine failure. In the real world, you would be able to use the collective, turn the pitch of the rotors "down" so while falling, they would be spinning faster without the engine, and then while nearing the ground, turn the pitch "up" so that the fast spinning blades would work as if they were powered and allow a safe enough landing (with some skidding, although that concept isn't yet in Arma for helicopters and needs to be) other than being in an inescapable crash.