The Propeller Constant Speed Unit is usually a centrifugal governor supplying or restricting oil pressure to the pitch change piston that is in turn connected to the blades. The PCU maintains the selected RPM by varying blade pitch, i.e. increasing pitch as the engine generates more power or decreasing pitch as engine generates less power to maintain selected RPM.
Throttles control the amount of power the propeller absorbs, not the speed of the propeller.
As engine power is reduced, the PCU will continue fining off the blades to try to maintain RPM. But there will come a time when the blades are at maximum fine, the PCU has no way of knowing this, it'll just keep fining off the blades to maintain RPM, all the way to max reverse if you let it.
So there will be a Beta valve of some sorts fitted, to prevent the PCU fining off the blades below a certain positive angle, the 'Flight Idle' stop, which may be a mechanical lump of metal or a hydraulic lock.
As the prop slows below governed RPM, any increase or decrease of engine power will directly affect prop RPM (by shoving more hot gas through the prop's power turbine) i.e. the throttles are now controlling prop RPM, you're now in Beta range. Selecting reverse will usually bring another governor into play to prevent the main PCU increasing pitch.
Ram the throttles forward, the prop speeds up until it reaches governed speed, the PCU will now start to coarsen the blades to slow the prop down and maintain selected RPM; you're out of Beta range again. throttles no longer control RPM, they're back to controlling power.