Event flow for head-scratchers?

There's a lot of talk of how the new Event Flow makes for easier communication between classes. This sounds swell, but nowhere is it properly explained. Every example I find concerns dispatching events within the same class or looped through a customEvent, but I need a way to tap into this Event Flow for backward communication.

I have a mainGUI document class that distributes my application subclasses. How do I set up a listener within mainGUI that gathers certain events from any of its children or grand-children? And how can I use this line to post object properties?