During autorotation, what forces primarily drive the rotor blades?

During autorotation, the rotor blades are primarily driven by the upward flow of air through the rotor system and the forces generated by the blades themselves. In normal powered flight, the engine drives the rotor blades, forcing air downwards to create lift. However, in autorotation, the engine is disengaged from the rotor system, usually due to an engine failure. As the helicopter descend....