What these code modifications do, is to create a single sign on across different, multiple domains for both phpbb and other sites/codes/apps to use.
These modifications use phpbb user tables, sessions, everything.
What it does is to set cookies for each domain through using iframes, which load some connect.php file that is placed on the other remote domains which needs authentication from phpbb. This is how numerous sites like facebook do it. Currently there is no other way of legally setting cookies for different domains than the one that is active. You have to have a page that loads up all the cookie setting codes from remote domains.
However the loading up of authentication page, the remote authenticator pages in their iframes, and the redirection is VERY fast, taking only 1 seconds or 1.5 seconds. just like how you see in facebook etc - or any other site that uses this method.
All files contain extensive comments, and also include configuration variables to change.