The slope of the cut is 4, so when you slide the left hand block down by 1 square, it moves to the right by 1/4 squares. The bottom row in the left hand piece is 3/4 of a square longer than the top row, so if you remove 1 square, it becomes 1/4 square shorter than the top row. So when placed in the top row it will line up with the left edge of the right shifted block.
Look closely at the column with the cut, after the square has been removed, that column is only a 3/4 width of a square.
The total area is reduced by 1/4 of a column by 4 rows = 1 square.
Damn maths always gets in the way of a free feed...
