Watching Conan O'Brian 2003-11-20 to start the new year, I learned from Billy Bob Thornton that the MPAA actually restricts the number of "thrusting motions" per shot in a sex scene.
I had a look at the Voluntary Movie Rating System and eventually know the american classifications now. Interesting to browse the IMDB and look at the "Certification"s. E. g.: Braveheart gets a just R for "brutal medieval warfare" whereas Lord of the Rings: The Return of the King gets a PG-13 for "intense epic battle sequences and frightening images". Yeah, only bad orcs get slaughtered, that's nicer.