courtesy of:
Hartzell
What is beta? Some constant speed propellers are equipped for beta/reverse operation.
Beta Range
Beta Range is any blade angle below flight idle (Hydraulic low pitch stop). Reverse
Reverse is any blade angle less than zero degrees. This blade angle produces thrust in a direction opposite to that of normal thrust. Such propellers are typically installed on aircraft with turbine engines and are used for to reduce landing roll.
Also some more info from ATSB
Here