Not knowing the PC6 in detail, but generalizing -
You select beta range with the power lever (aka throttle) by moving it below Flight Idle. In this range the throttle directly controls not only engine power/torque/fuel flow, but ALSO prop blade angle. That's the engineering definition of beta control mode.
This means the throttle is linked to two control cams, engine power (Ng/core speed on the PT6) and prop angle, since in beta the prop governor is disengaged. The two cams may not be perfectly matched, so prop rpm may not be a specific value, but "close enough for government work" as my dad used to say.
And of course, most a/c types have beta mode inhibited/blocked in flight.