It will close any open app not listed in the app group that you just clicked on. So, for example, if I don't have Excel in any of my app groups and I click on an app group that has Emacs and Terminal, then Excel will be closed.

It only closes apps that appear in the dock. It won't close dropbox or alfred or any background processes.

edit: And I also want Alfred integration :) I was thinking it could create .foreman files which you could then open with Alfred. Foreman would then handle it the way it handles clicking on an app group now.