There is one pitch change piston inside the prop hub which is connected to all the prop blades; when its at its full extension you've got feather. When it's at it's full retraction you have max reverse. Beta is just a name for a certain range of the pitch change rod. You could just as easily call that range 'Pitch Change Rod In The Lower 40% Of Its Travel Mode'. Beta's a lot easier to say.
How do you get in it? Power from the engine has to be sufficiently low to allow the prop to slow down enough so that the PCU governor is commanding full fine. You then need the flight idle pitch stop (which is stopping the Pitch Change Piston from fining off the prop below flight idle) to be withdrawn. Usually this will be linked to the weight on wheels system. When that stop, and it is a mechanical lump of metal physically stopping the pitch change piston moving, when that is withdrawn then the pitch change piston can fine the blades through 0 degrees and into reverse. The B1900 for example has a beta valve. This creates a hydraulic lock in the pitch change piston, stopping it going beyond a certain distance which corresponds with the blade angle for flight idle.