I read over the documentation several times and have been having a hard time trying to figure out what is going on with the function. I'm more and more confused after looking at the documentation, looking over the source code as well.

add_filter($tag, $hook, $priority, $args);

it seems to me the new function extends the parent function. What puzzle's me is what parts of the hook becomes overridden. in some examples in the documentation i see that some variables are replaced with the $args in your new $tag.

I almost understood it all here: http://www.andrewnacin.com/2010/05/18/rethinking-template-tags-in-plugins/

but then i couldn't figure out how you pass arguments and which eventually get overriden.

thanks in advance.