The heart of an automatic watch is usually a rotor with ball bearings. When the rotor, the so-called balance wheel, moves, it generates a torque due to inertia and gravity, which is used to wind the movement. In most movements, the rotor moves only in a semicircle. In this way, a slight rotation of the watch is sufficient for a complete 180-degree rotation of the rotor. To prevent the mainspring of the watch from being overwound, a so-called slipping clutch is used, which in this case separates the mainspring from the winding mechanism.
A power reserve is created by the tension of the spring and ensures that the movement continues to run for a while even without being moved. As a rule, depending on the model and manufacturer, the duration is between 30 and 60 hours. Watch winders can be used to store automatic watches for a longer period of time without the watch stopping. Such devices are therefore particularly often used by collectors or owners of several automatic watches.