How to set default controller for sub-directory in controllers

[eluser]gunnarflax[/eluser]
Hi! I have a multilevel-directory structure of my controllers directory. The user guide says that this is fully possible, all I have to do is set a default controller for the sub-dir in the config. The thing is that there is only one key called 'default_controller' and I can't find anywhere how the extra default controller should be named. What should I call it? And how do I need to configure my routes?

Thanks!