Here a small plnkr to show it in action -
ParentCtrl is a typical Angular controller function but you'll notice that it's not registered as a controller. It depends on $location service while the ChildCtrl does not. ChildCtrl gets hold of injector and calls invoke on it passing the "parent" reference (ParentCtrl in this case) and its own reference as second argument. The invoke function does the magic of "mixing-in" the parent behavior into the child.
I have found this technique incredibly clean and useful so far. It helps making controllers DRYer and reusable. Hope this helps you too.