Well, at the beginning of the semester, MIT offers 3-hr advanced standing exams (ASE) in a variety of courses (e.g. introductory chemistry, introductory biology, etc.). If you can pass, then you can skip the class. This is used because of the fact that MIT typically does not offer GIR credit for AP exams or classes otherwise taken in highschool. You can go on ocw.mit.edu to get a better idea of the kinds of exams you might expect on the ASE, though I hear that the chemistry ASE is wildly more difficult than the actual class's final.
Anyways, "skipping semesters" really just means that you can take a lot of advanced classes in senior year, unless you're gunning to graduate in three years (which has been done before).