This term refers to a mechanically powered watch that is wound by the motion of the wearer's arm rather than through turning the winding stem ( manual mechanical). In response to this motion, a rotor turns and winds the watch's mainspring. Most automatic watches have up to 36 hours of power reserve. If an automatic watch is not worn for a day or two, it will wind down and need to be wound by hand to get it started again. Click here for automatic watch instructions.