Answer by Thomas Zerbarini:
First, the autorotation maneuver is an emergency descent maneuver and is not a gradual descent. It is a rapid controlled descent.
Gravity is pulling the rotorcraft to the earth. As the helicopter descends rapidly through the air, what fast moving air traveling up through the rotor blades allow the blades to store energy and faster. To do this, the blades pitch angles are reduced "flattened." As the rotorcraft nears the ground, the pilot increases the pitch angle of the rotor blades to increase lift and used the stored energy of the rotor blades to softly land the helicopter.
Here is a video showing the aerodynamics of autorotation:
Here are several videos showing pilots demonstrating and practicing autorotation. Notice the fast descent of the helicopter followed by the high pitch angle of the rotorcraft as it nears the ground. This is the pilot gradually increasing the pitch (using the collective) as he/she nears the ground and pulling back on the stick (cyclic) to arrest the rapid descent.