If you fix the collective and have manual throttle (i.e. no governing of rotor RPM), and change airspeed the rotor RPM will increase for the following reason.
The power being put into the rotor system is constant, and the increased airspeed will increase the mass flow of air through the rotor disk. The engine is providing power to overcome the drag of the rotor blades, and the increased mass flow of air through the disk will decrease the drag on the blades.
Viewed the other way around, if you have a governor that maintains rotor RPM constant, and hold the collective fixed and increase airspeed, the power to keep the rotor RPM at a constant value will decrease, and you'll see a reduction in manifold pressure or torque.