One other point that seems to be confused in an otherwise thorough post:
Feather should not be thought of as the "Zero Degree" position; remembering that the "Blade Angle" is relative to the direction of prop rotation and not the airplane's forward motion, feather is actually closer to 90 degrees. The propeller does NOT pass through a feathered position to reach beta angles; feather is the MAXIMUM (mechanically limited) angle the blades can acheive. As you enter Beta range, the prop will have gone the other way, through fine pitch. Logic says that to reach a maximum-drag blade angle (Reverse), one would not want to pass through the MINIMUM drag configuration (Feather).
Using the Dash-8 as an example, the Beta Metering Valve activates with the Power Levers just slightly above Flight Idle, maintaining the blade angle at 17.5; this actually occurs in flight. On the ground, pulling the throttles past the Flight Idle gate will continue to push the prop farther into Beta, REDUCING the blade angle more and more until it reaches the "Disc" position, which has the blades at 1.5 degrees, almost flat relative to the motion of the airplane. During this stage the engine is idling, only putting out enough power to keep the RPM from decaying into the restricted low-RPM range. This "Discing" is the big drag referred to above, and in this position the prop actually throws some air SIDEWAYS (go stand next to one in Disc sometime if you don't believe me!). Passing another stop into the "Reverse" range, the prop angle goes negative, decreasing all the way to -12 degrees; simultaneously the ECUs will increase engine power, which DEFINITELY blows A LOT of air forward (again, go stand in front of one sometime!).
System-wise, the Beta valve actually controls regulated bleed-off of oil pressure in response to a blade-angle feedback signal from the prop, doing so to maintain a specific blade angle commanded by a linkage to the Power Lever. In the extreme lower range of Beta an additional valve (Reverse Valve) opens to allow unmetered oil pressure into the fine pitch side of the prop pitch change mechanism, which is then regulated by the Beta Metering Valve as described above.
Operationally, in the "Alpha" or Governing range of the prop, the PCU or governor varies blade angle to maintain set RPM; in the "Beta" range, the governor is bypassed and the Power Lever directly controls blade angle to adjust drag for ground (or sometimes flight) operations.
Again, the example above is for the PW 121 in the DHC-8, which is a free-turbine engine. Similar principles apply to most others. Hope this helps!