He means if you were to look with your regular vision you would be looking at a spot that makes the center of a book. Yet with using photofocus you would be looking past that point so in you field of vision is both pages of the book.
You could look over the top or bottom edge of the book but Paul prefers to train you to look through the center right from the start because on some larger books looking over the top or bottom does not comfortably being all the text on both pages into your field of vision.
Personally I use left eye left page, right eye right page because thats where they land if I look through the center of the book
Alex
