Probably a silly question

[eluser]WanWizard[/eluser]
When you look in the loader code, you'll see that first the orginal class is loaded ('Class CI_Router'), then a check is made for the existence of an extension ('Class MY_Router extends CI_Router'), which is then loaded.

Based on the existence of the extension, it then does a '$CI->Router = new CI_Router()' or a '$CI->Router = new MY_Router()', thus making sure you can access the router class consistently, whether or not an extension is present.