Refer to figure.
Variable pitch propeller: designed to keep a set rpm throughout the entire flight range. The propeller controls the engine at this set rpm through a device called a Constant Speed Unit (CSU). Any variations in engine power output, which would normally produce a change in rpm, will be sensed by the CSU that will alter the propeller’s pitch angle to keep the rpm constant. For example, in cruise when power is increased, the propeller pitch angle will be increased by the CSU and move the blade towards the coarse position. To avoid an engine overboost the RPM will be increased before increasing the manifold pressure.