I assume it is connected to http headers; http request; cgi; get; post HELP HELP and http_cookie, cookies, extracting from $ENV variable set cookie question. It certainly looks like he is trying to be able to give a fake URL where someone will hit his login screen, he collects the name and password, then he redirects them to the real site with none the wiser.

However the basic idea is one which would be useful if a collection of sites wanted to allow someone to login to one of them and then "carry" that login to other sites in the ring. But I would expect someone who wanted to do that to be describing what they are doing rather differently. (Note the "tricking the site" description.)