Under the Loupe/An Introduction to Automatic Winding Mechanisms
Automatic Movements
Like simple wound movements, automatic movements are driven mechanically by a spring. Timepieces which fall under this category, however, are fitted with a mechanism that translates the natural, kinetic motions of the arm into into potential mechanical energy stored in the mainspring. Such a mechanism, usually, works via inertia and the principles of centrifugation.
[edit] How It Works
Attach a piece of stone (or your own wristwatch if you like) to a string, hold the other end (tightly to avoid unnecessary damage to property whatsoever in any form.), and then allow the object to revolve. Similar to how this object revolves by a string by the motion of the hands, the mechanism in an automatic watch has a weight (also referred to as a rotating mass) that revolves and winds the spring through a series of reduction gears. In the picture above, for reference, the weight can easily be spotted - it is the large piece of metal with the text "SEIKO TIME CORP" etcetera stamped on it.
There is great debate over whether it is most ideal for automatic winding mechanisms to wind the mainspring when the weight revolves both clockwise and counterclockwise (bidirectionally), or whether it is best to have the mainspring wound when the weight moves in one direction only (unidirectionally).