When the squares are taken out of the frame you can’t see them all, some are out of shot. The answer is out of shot. This trick can’t be done if all the squares are visible and in shot all the time.
Simple geometry – it is not a true triangle so if you reverse one angle with the opposite it is very eay and he could have shown it 5 to 8 times with the same pieces. Taught in 9th grade when I was in school in 1971..I wasn’t a great math student either.
planes and angles – used heavily in the late 70′s and early 80′s to build those unique looking business towers/skycrapers we called them