The driven region is the part of the blade that is producing more drag then lift. as you lower the angle of attack, you are decreasing the amount of drag that the blade is producing.
So, as you drop collective, you are producing less lift, but also less drag. the driving region increases in size and rpm's increase because there is more force driving them then there is drag slowing them down. Of course, the opposite is true when you raise collective. More lift, and more drag. The driving region shrinks as the driven region grows resulting in rpm's being lost.
If you are autorotating with a constant rpm, then the driving region is spinning the rotors up at the exact same pace as the stalled and driven region are slowing it down.
(i think
)
by the way, welcome to the forums! (although I just noticed you joined before I did...oh well)