Refer to figures.
In steady speed autorotation, the helicopter is descending vertically with a constant forward airspeed and a constant rotor RPM. The rotor blades are in autorotation, meaning that they are being driven solely by the upward flow of air through the rotor disc.

To maintain a steady descent rate and airspeed during autorotation, the Pilot must adjust the collective pitch to control the lift generated by the rotor blades. If the Pilot wants to increase the thrust and reduce descent rate, he/she can increase the collective pitch, which increases the angle of attack of the rotor blades and generates more lift.

However, the rotor RPM at this stage will decay due to the increased rotor profile drag and the centrifugal forces will decrease.

The combination of the increased lift and the decreased rotor RPM will cause the blades to rise, until they reach a position where their upward movement is balanced by the centrifugal force produced by the blades’ rotation. When raised like this, the blades are said to be coned upwards.