I asked what would happen if torque were too low to drive the propeller at the selected rpm. The answer, based on the information I gave you, was that that the governor pilot valve would open fully, maximizing pressure in the propeller hub, driving the piston all the way forward, and the blade angle to -15 degrees. As you can imagine, this would not be desireable. If the pilot reduces the power to land, she wants to descend to the runway going forward, not in reverse. This doesn't happen, and it is the job of the beta system to ensure it doesn't.
Three rods extend from a feedback ring at the rear of the propeller to the outside of the propeller dome. When the propeller dome moves forward past the position corresponding to +21 degrees blade angle, the linkage from the propeller dome begins to pull the feedback ring forward. This movement is transmitted to the beta reverse valve, which begins to close. At less than an 11 degrees blade angle, the beta reverse valve will fully close, not allowing oil pressure through the beta valve. The governor pilot valve will be wide open, but that won't increase oil pressure until the drop in oil pressure allows the springs and counterweights to drive the blades to a coarser pitch, opening the beta reverse valve, and holding the blades at the ideal idle angle of 11 degrees, until torque increases. As a result, when the propeller is in beta range, the power levers control fuel flow and blade angle.
If the pilot really does want to put the propellers into reverse, useful for parallel parking (I am NOT kidding: you really can parallel park these guys), the beta reverse valve can be deactivated. The pilot does this by twisting the handgrips of the power levers, kind of like you may have to push a button or apply downward force on your car gearshift to shift into reverse. Twisting the handgrips allows the beta reverse valve to open, so that blade angles less than 11 degrees can be obtained. As the power levers move aft from that position, they stop controlling the fuel flow and control only the blade angle through a null zone between 11 and -2 degrees blade angle. As the power levers move back past the position that selects -2 degrees, they start increasing fuel flow--normally you push power levers forward for more power--so that pulling the levers further back allows you to increase backwards thrust, either to reverse into your parking space, or to slow the airplane down more rapidly after landing. A blue (or possibly amber, depending on the model number) light turns on to show that the propeller is in beta mode, with a blade angle less than 9 degrees.
The manual says, literally on the first page, even before the table of contents, that you must never ever ever do this in flight under any circumstances. This of course means that loads of people do, but that the company isn't going to be held responsible if bad things happen.