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...