Finding the missing view (video)

• https://www.youtube.com/watch?v=mt6GDGuk1NM

I'm trying to follow this guy's explanation on finding a missing view starting at 7:30. What I'm really struggling with is when it get the point at 9:20. He is referring to the top two rectangles on the front view and their corresponding edge on the right view. He's asking, "How do we determine which vertical edges on the right view belong to those rectangular surfaces on the front view.

He says the vertical line on the right must be in front of the one on the left. Then he says the left vertical line must be behind the front vertical line which is the right line. I'm looking at the drawing when he's saying this but I'm in a tailspin at this point.