Move your head close to the screen. Do the perfect blue squares appear to be distorted? Absolutely… this is due to the black lines in the back moving and twisting it in your mind. You’ll also notice that the backgrounds appear slightly different shades, as you look at them. To me, the lines that are moving rrom the top left to the lower right are slightly darker than the ones moving from the bottom left to the top right! They also “scintillate” which means that they’ll slightly flash and appear to slightly move.
The main part of this illusion is that the crooked blue squares are not really crooked. They’re perfectly square! What an eye trick!