Reaction control wheels are used in spacecraft to control
orientation in space or to correct small trajectory
errors. Reaction wheels are basically just flywheels, and are
normally aligned along three axes perpendicular to each other, so that
each may absorb the angular momentum component about its axis.
During the flight of the spacecraft, conditions may arise where the
reaction control wheels are spinning as fast as they can, and can no
longer be used to absorb spacecraft momentum. This condition is called
momentum saturation, and requires the spacecraft to perform a maneuver
called a momentum dump or an angular momentum desaturation (AMD). This
maneuver just means that the spacecraft uses its thrusters to cancel
out the momentum in the reaction wheels. Force is applied to the
wheels to slow them, and the rotation induced in the spacecraft is
canceled out with the thrusters.