An automatic watch utilizes a weight which moves with the motion of the wearer, and was at first a pendule type, followed by various designs that included 'wig-wag' wristwatches as well as 'Bumper' type weight systems.
The most common today is the rotor, or oscillating weight type. Not all automatics are created equal, I could fill pages with the history of the variations, but for the purpose of the blog, I'm just going to describe what happens inside your modern automatic watch, especially when you bring it in for service.
Generally, during a full service or CTR, the mainspring is one of those parts that gets replaced as a rule, regardless of its appearance. (For the purposes of this blog, I am talking about modern automatic watches here, which means mid 20c to present)
As I have pointed out elsewhere, overwinding is a myth, you wind your watch until it stops, if it is a manual wind. For an automatic, the motion of the wearer and subsequently the rotor inside, is what winds the mainspring. At some point, the watch is fully wound, so now what?
Adrien Philippe (of Patek Philippe) developed a mainspring with a slipping bridle. This end is not attached to the wall of the barrel as it is with manual wind watches, it slips on the wall of the barrel against a film of braking grease.
If the wearer is so active as to make the watch wind itself completely, the spring will slip, so that no damage is done to the mechanism. These springs are usually described in watch literature as 'unbreakable', but they do break occasionally. Usually the bridle end breaks off, but sometimes the spring will break right near the centre, at the winding arbor.
So you wear your watch, the rotor spins around, and the watch winds itself. Green technology at its finest, no batteries or capacitors to worry about.
The automatic mainspring looks like an 'S' shape when it is uncoiled, and is a very specially engineered spring. The outer coils are curved the opposite way of the centre, placing extra friction agains the barrel wall via the bridle.
There are no fewer than 3 different types of lubrication in the barrel of a watch, and each does its job to mitigate friction. The barrel itself is smaller than a dime in most men's watches, even smaller in ladies watches.
So that's it- your watch gets the full treatment and the spring is now brand new, ready to keep on powering the movement. A well cared for watch can last hundreds of years. Here is the spring uncoiled, next to the arbor around which it is wound: