Ian, your explanation is better than any I have heard. I am having a problem with your explanation (and all others including mine) in explaining how the "flare" goes outside of the frame area into the border. This area is normally masked completely and would even be protected from light via the method you give. So, I'm not sure. Maybe my mind picture is off. IDK. I have changed my mind twice by reviewing the photos posted here.
I do have an RZ and had an RB and so I am interested in the explanation.