A perpetual, or self-winding, watch keeps itself wound using a component called a rotor which is moved by gravity when the arm swings or moves and is connected to gears that translate the movement to circular motion within the watch. These watches usually generate enough energy from movement to keep themselves running for one and a half to two days even when sitting still.
The rotor is the essential component in a self-winding watch. It is a large semi-circular piece of metal within the watch that connects to various gears. As the wearer moves their arm, the rotor is pulled around by gravity and pivots. This motion is transferred to a series of reducing gears and reverser gears. These gears eventually transfer the energy to the mainspring, a spiral mechanism that controls the gears that move the hands on the watch face.
Each movement of the arm winds the gears only a tiny bit, but the overall amount of movement of the wearer's arm is enough to keep the watch powered. So much energy is generated, in fact, that self-winding watches also contain a slipping clutch device that stops the watch from building up too much tension and breaking the mainspring.