^As you can see, your mind AUTOMATICALLY FOLDS the red papers UP so that it's facing you – and the whole thing makes sense even before watching the video.
I used this rule many times when building things: it's how you make SQUARE corners.
Measure one side, measure the other, measure the diagonal between the corners. Make sure the diagonal matches up with across x across, + down x down = diagonal x diagonal. But a picture is worth a 1000 words. And a video is worth a million words.^