tl;dr; There are some foreign works that are still copyrighted overseas, but the copyright has expired in the US. This ruling allows the Congress to reinstate the copyrights for particular works so that the copyrights apply for the full length of their term in the local country that they were originally created/copyrighted in.
Is there any limit to what it does past this? If the intention is to fit in with the worldwide agreement by doing what you describe, it should be written in a way that it can't be used to extend Mickey Mouse's protection or something shortly before it.
