Automatic Movements
An automatic movement requires the use of a rotor. This is an oscillating (rotating) weight attached to the movement, allowed to spin within the case while the wearer's wrist moves. The rotor is linked to a combination of gears to the mainspring (aka Balance Spring). As the rotor rotates, it winds the mainspring to give the watch kinetic energy. The rotor is engaged with a clutch to disengage from winding when the mainspring is completely wound. Power reserve is the term used to indicate, in hours, the amount of energy that is stored in the movement. So, a watch with a 48-hour power reserve implies that it should work a full wind for 48 hours before the energy is exhausted.

1. Movement of the wrist allows the rotor to spin, this winds the mainspring. (typically turning the crown will also wind the mainspring.) 
2. The gear train flows the kinetic energy to the escapement in the movement.
3. The escapement disperses out the energy into each of the regulated parts.
4. The balance wheel uses this energy to beat back and forth at a regulated rate.
5. Every certain number of beats, the dial train transfers the energy to the hands of the watch. 6. The hands then advance.