It immediately closes the pop-up/pop-unders popping out while you surf.
It catches links from flash too.
You can add your own filter and enable it to close any other windows by choosing 'Options' from the add-ons menu in Firefox.
Why 'a rude popup-popunder Killer' ?
It checks the first 30 characters of the window's URL and if it contains the excluded terms it closes it immediately without further checking!
It is very simple and effective, but can cause problems if you add the wrong keyword.
That is why I don't provide a handy menu to add websites on the fly.
So, by having to go to Tools->addons->options->killjasmin->options and add your filters gives a bit more safety and it's not that hard...