I will hope this may give you some answer to your question, I take it from your question that you are not really familiar with the ways and controls of Helis, however as from my small knowlege of this, it the following,
If you enter Autorotation, it is because of two possibilities,
1. everything has gone quiet, it then is an imperitive to enter Autorotation.
2. You need to get down quickly for whatever reason, so you would enter under your control( however I dont know many who would do this)
The control inside the craft that imparts or controls rotor pitch is the collective, (the cyclic controls the disk) this not only controls rotor pitch but also blade and power attitude Ie course(high power) or fine pitch(low power) of rotor blades, the angle of attack is the finess of presentation to the atmosphere of the leading edge of the rotor blade, and this is also a task controlled by the collective.
If things go quiet( engine failure for whatever reason) it varies from type to type but your immediate re-action is to close or pushdown on the collective, this puts the rotor into its most fine setting, to give a slightly easier example, if a blade at rest = 0,or fine pitch(low power),.. course pitch would be 10(high power), the collective as such has a bottom and a top limit set by the manufacture, whilst it is quite possible to over pitch ie=10, open the face of the rotor so much that you are demanding more than is possible from that rotor/engine or turbines it is not possible to present a negative pitch for this would have dire consequence's on rotor blade/performance, and could lead to blade intrustion, this would spoil your day, nor is it possible to push the collective past the manufacturers preset closed position, IE =0
So if you are unfortunate enough to have to select autorotation, you as the pilot holding the collective, must then control the descent of your heli by judicial use of the collective, and craft attitude with the cyclic, with great care the pilot will stop what is called an overspeed of the rotor which in itself can be structurally dire and cause problems.
A helicopter flys because the engine linked to the rotor produces enough energy in the form of downwards air flow to allow the craft to overcome gravity, in the hover and in forward flight there is always some element of downward thrust from the engine through the rotor(s), remove the energy from the engine and you then have to rely rotor inertia and the upcomming atmosphere or airflow to give you the rotor authority to hit terra firma under controlled conditions.
As I am waiting for my wife to do something for me, I have had time on my hands, this I fear is my longest and possibly most boring answer ever to any thread, I hope it makes sense to you, I am not an English master so forgive incorrect spelling and grammar
