Rotor efficiency vs airspeed
For a given collective setting an increase in airspeed increases the rotor efficiency.
If you maintain a constant pitch angle with collective and accelerate to a higher airspeed, the rotor efficiency will increase.
The power required to maintain the collective setting reduces. The RPM will then want to increase due to more power being in use than required.
On a governed machine (R22/44 or turbines) the governer senses the RPM increasing and reduces the power (MAP or TQ) to maintain RPM.
On a non governed machine (H300) the RPM will increase and the throttle will need to be manually reduced.
The opposite occurs if airspeed is reduced due to the rotor efficiency reducing.