The vertical part of the ‘T’ (right) changes its length. Click on it, the movement will stop, and you can now drag the size until you are satisfied that the vertical and horizontal parts are identical in length. Instead of dragging, you can press ‘+’/‘–’ on your keyboard.
When you now press the “Show Result” button, the length in percent is displayed and a rotated version of the horizontal bar overlays the vertical one.
The top left pop-up menu allows to test this for various rotations. Select a new orientation, then click somewhere on the T, and adjust using the ‘+’/‘–’ keys.
Does the strength of the illusion depend on orientation?
Most explanations don’t convince me particularly (but I’m open to suggestions, of course).