Weads, I think I now understand what you are asking.
In a vertical autorotative descent, the air is slowed by the rotor disc (extracting energy from it) and the Nr will be say 100% with the lever probably on the bottom stop. The effective rate of descent (as experienced at the rotor disc) is less than the actual rate of descent as the column of air is slowed and can only pass through the disc at a certain rate - an equivalent parachute of the same size was a phrase used many years ago in Brit Mil teaching.
Now, when you increase speed, you are moving the aircraft into a faster moving mass of air (not slowed by the rotor) so the effective rate of descent goes up, increasing the Nr and usually allowing you to add a little collective to keep it in the green. However the actual rate of descent reduces.
Your text books may refer to factors A, B and C where A is the tilting of the disc that reduces its area to capture the moving air and so increases RoD, B is the change in inflow angle from increasing speed which also increases RoD but C is the effect of moving into faster air which increases rotor thrust and decreases RoD. So Factor C outweighs factors A and B.
This keeps happening until you get to your best auto speed (60kts I think for R22), beyond which the disc tilt and inflow angle outweigh the increase in airflow - lots of drag now comes into the mix requiring more nose down to overcome.
Some aircraft have a VNE in autorotation because the combined effect of factors A and B (and their effect on the inflow angle) means that the inflow angle gives too much rotor drag and not enough driving force to keep the Nr high enough.