On a windmill, fluid power, specifically hydraulics, is used to control the pitch. In some cases, a hydraulic brake is used to stop the rotor in emergencies.
Pitch control keeps the rotor from turning in winds that are too high or too low to produce electricity. Pitch-controlled wind turbines have an electronic controller that checks the power output of the turbine. When the output is too high, the blade pitch mechanism is ordered to pitch or turn the rotor blades slightly out of the wind. When the wind drops, the blades are again turned back or pitched into the wind. To maximize output for all wind speeds, the computer will pitch the blades a few degrees whenever the wind changes to keep the rotor blades at the best angle to generate the most power output.