Now I'm not an instructor, and my explanation may not be text book or 100% correct, but I'll throw in my two cent's worth. (I hope it's close to being right, though). But Overpitched is correct.
In autorotation, the key is keeping the RRPM in the 'green range' because it is at this speed that lift is produced most efficiently and that's what you want to do; keep the rotor producing as much lift as possible to keep you in the air (endurance) or to get you travelling across the ground fast (range). In either case, RRPM is key to ROD and/or range.
By pulling in pitch, you increase AoA and the driving region of a rotor blade moves further outboard (and that's OK) but it also causes the stalled region of a blade closer to the blade root (where Angle of Incidence is highest due to wash out) to increase and the point of equilibrium to move outboard because the resultant angle of attack also increases to the point where more of the blade stalls. (Clear as mud, so far?)
RESULT: Pull in pitch > more of the blade stalls > less driving force and more drag force > RRPM slows down to outside green arc > less lift.
By letting RRPM increase too much (say in a turn during a 180 auto), then the opposite occurs whereby the driving region moves inboard and the stalled region decreases which sounds good, but the DRIVEN region also increases and this region doesn't produce useful lift.
RESULT: Keep pitch the same > increase RRPM because of turn > driving region moves inboard > stalled region decreases > driven region increases > RRPM increases to outside of green arc > less effective lift.
Most effective speed is usually in the bottom of the drag curve (VminD) which for a Chinook is about 80 knots. This is the speed where the rotors have the least work to do to overcome the various forces of drag... and so lift is the most effective. This is great for keeping ROD to a minimum.
Most effective speed to cover distance is 20 knots faster at 100 knots. ROD is a little higher because of the attitude of the aircraft and the resultant change of airflow through the rotor disk but the aircraft will be able to travel further for the least increase in ROD.
In the Schweizer (now Sikorsky) 300, the VminD is 55 knots so that's the speed you're looking for in an auto.
So short answer: Keep the RRPM in the green arc and sweat on your attitude and thus your IAS for minimum ROD or maximum range.
Anyone who thinks I've got it wrong, please feel free to correct me and help me see the error of my ways.
Hope this helps.