Ok, so I'm REALLY, REALLY color-blind... more than usual for men. But I thought the photo looked cool.
Basically, from what I understand, "S's" are connecting the intersections constructed by the rule of thirds (roughly) with curved lines.
Rather than building a photo as a single or series of "L's" or an uneven "X," according to the rule of thirds (connecting the intersections with straight lines), the photo is composed by positioning naturally occuring attractive curves.
Look for the "S" or reverse "S" in the landscape photos you like on the forum. Quite a few of the really good ones (nothing is constant) will have the "S" shape, and your eye will naturally follow right along with it and bring you into the photo (maybe).