RRPM increase in flare
We can also leave out Coriolis and Einstein...
Compare the flow in level flight of the heli with the level flight of a plane (forget the rotating part) : at sufficient high blade speed (outer part of the rotor) the wind blows in horizontally to a foil with a certain "positive" AoA, just as it does with an airplane wing : this creates lift and drag just as it does with the plane (the only difference is that drag becomes torque)
Now the flair or autorotative state : put the heli blade flat or even slightly negative and let the wind come well from underneath. With respect to the wind vector lift now becomes a "pull" the effect of which can become larger then the drag and this can ultimately drive the rotor. This again is the same for a plane: putting the plane in a descend will create the same difference in geometry (wind blowing from below) that drives that plane as a glider.
(sorry no time to make the drawings that go with the text above, but hope it helps)
d3