Refer to figure.
Nowadays turboprop engines deliver very high power in flight. Through the Constant Speed Unit, the propeller controls the power output by varying the propeller blade pitch angle to maintain engine rpm. The blade pitch angle range required in flight is known as the ALPHA control range, and the power lever positions corresponding to the ALPHA range are between flight idle and maximum power. However, while maneuvering on the ground, the power required for a turboprop engine is pretty low. Therefore, the controls have been adapted such that the blade pitch angle (therefore the thrust produced) is controlled directly by the pilot, overriding the CSU. This ground range is known as the BETA control range.

The flight crew will select a blade pitch angle by moving the power lever fully aft and lifting it up. The first detent of the BETA range will correspond to the ground idle detent that will allow the pilots to produce minimum level of thrust, just enough power to taxi the aircraft. If the lever is pulled full aft of the BETA range, the pilot will lower the blade pitch angle such that the maximum reverse power is produced (=negative thrust). This will usually be the case at landing in order to assist the aircraft’s deceleration.