It's a rather complicated way to ensure that all points have y coordinate between 2 and 4, thus cutting all lines that would extend out of the "writable" two sq

It's a rather complicated way to ensure that all points have y coordinate between 2 and 4, thus cutting all lines that would extend out of the "writable" two squares. That expression equals zero if and only if 2<=y<=4. (edit) If you try "L", the formula contains a product member (x-2) that would produce a vertical line whose points all have x=2, containing the vertical segment of "L". The final string excludes that.