The rotor and mainspring
The rotor is a weighted component that turns as the watch moves. Through a reversing or winding system, that motion winds the mainspring. The mainspring stores energy, while the gear train, escapement and oscillator govern how that energy is released to display time.
Different automatic calibers wind in different directions and have different winding efficiency. The rotor may be visible through a display back or hidden behind a solid case back. In either case, normal movement of the wrist is not guaranteed to keep every watch fully wound, especially if it is worn only briefly or has limited motion.
Power reserve and winding
Power reserve is the approximate running time after the watch is fully wound and then left unworn, under specified conditions. It varies by caliber and can be affected by use and condition. A watch that stops after a couple of days may simply have reached the end of its reserve, not failed.
If the watch is stopped, follow the specific manual for winding and setting. Many automatics can be wound through the crown, but the safe method and number of turns differ. Do not force a crown, operate calendar settings during a restricted changeover period unless the manual permits it, or shake a watch aggressively to start it.
Winders, accuracy and care
A watch winder is a convenience, not a necessity. A stopped watch can be wound and reset when next worn. A winder should match the movement's requirements and should not be treated as a substitute for service; continuous running adds operating time to the mechanism.
Keep an automatic away from strong magnetic fields, impacts, extreme heat and unapproved opening. Water resistance depends on intact seals and correct use, not on the movement being automatic. Have the watch inspected if it develops condensation, stops repeatedly or changes rate substantially.