"Self-winding watch" made me think, how in the hell? I figured maybe it was actually using some fancy bit of electronics available these days that harnesses the energy from vibrations.
But alas, no. It really is a self-winding mechanism, that harnesses the energy from your moving arm and gravity. Observe:
http://static.howstuffworks.com/mpeg/q285.mpg
http://electronics.howstuffworks.com/question285.htm