Asked my nephew in Australia, and this was his answer. It’s all about the angle cut and moving the piece on the left to replace the piece on the right. By doing this you reduce the area (blocks) by a row.
The original rows was 8, by moving the left side to the right, it reduces the rows to 7.
So now you know.!!
Anything else you want to know??
Its mass versus area. The piece left over if cut correctly would fill out the smaller squares and make it a perfect mass again. The way to test this is to draw an 8x5 grid then cut it out and you will see what I mean.
Because it was cut out as a square, if you cut a diagonal line through something then you can alter the overall lenght by sliding along the diagonal. The rest was cut it a fashion to fill the rectangle and make it look complete with one square over. Try it with a piece of graph paper and you will get a better understanding of it.