In the last post in this series I explained how private forum permissions work in phpBB2, and then how an Auto Group MOD might make use of these permissions to dynamically grant permissions for a board member to view a private forum. In other words, the forums are managed by the normal phpBB2 security process and are hidden until I am added to the proper group. Then I mentioned how the Forum Auth by Post Count MOD works in an opposite direct. Forums are not private, they are public instead. The MOD hides these forums until they should be visible.
It does not override or replace the standard permissions system. It provides a new layer on top.