A self-winding (automatic) mechanical watch movement uses an oscillating weight (also called a “rotor”) which oscillates back and forth based on the natural motions of your arm/wrist and winds the mainspring which powers the watch hands and other functions.

The oscillating weight rotates both ways, and power is either generated in one or both directions depending on the design. Bi-directional rotors such as the type used in modern Rolex sports watches wind in both directions which typically means the watch winds faster compared to a unidirectional rotor.