It's similar to a fixed wing airplane really, except that it's happening in a circle. It's done by reducing the rotor blade pitch to the minimum when power is initially lost, by lowering collective, to ensure the blades are able to maintain airspeed (in the form of rotor RPM), then adjusting pitch with collective as necessary to keep RPM in the green arc.
Take away the motive force, the engine and prop in the airplane, or the engine torque acting on the blade root in the helicopter, and you have to lower AOA to maintain flying speed, then adjust AOA to maintain the desired speed (or in the helicopter, RPM). In autorotation, the flow switches from down through the rotor to up through the rotor because that's what it takes to maintain a flyable AOA.
The autorotating rotor is being driven around by the same forward thrust component of the lift vector that moves a gliding airplane forward. The autorotating rotor is just like two gliders passing each other and who's wingtips stick together because the tips were made of Velcro, forcing them to glide in a circle centered around the joined wing tips.
Because they are rotary wings going round, the AOA is high near the root and lower near the tip because the blade speed increases going outboard. Lift is perpendicular to AOA, so the thrust component of the lift vector is stronger near the middle and root than near the tip because the lift vector is canted forward, and the vertical lifting component of the lift vector is stronger near the tip because the lift vector is nearly straight up.
This is why they say most of the lift is coming from the outer part of the rotor disc and most of the driving force is coming from the inner and middle section of the disc (inner to mid span on each blade).
In an autorotation landing, inertia is used to convert forward energy of the blades to additional lift energy. As you come out of the autorotation after the landing flare, you go back into "powered mode" you might say, by pulling pitch, changing the flow through the rotor from upward to downward as when the engine was running. Except the "power" you are using isn't engine torque, it's just the inertia of the rotor blades. You have a couple of seconds to use this inertia before the RPM decays and down you come.
It's a bit like settling a gliding airplane on the ground by pitching the nose up and trading inertial energy of the plane into temporary increased lift.