What to see?
All there is to see are: a white line separated from other colored lines by a gray field. You probably think inevitably which of the colored lines is a continuum of the white one.
What to do?
To realize which one is continuing the white line you have to drag the white line with the mouse until they will be in a horizontal position. This will cause the angular displacement illusion to disappear.
If you will start to increase the angle of the lines away from the horizontal in a gradual manner you will notice that the illusion will increase.
What else can you do? You can drag the right side of the gray bar, thing that will make the middle vertical bar wider. The length of the bar will also produce a new perception of the angular displacement. The largest displacement is obtained when the middle bar is at its maximum width and when the lines are at the maximum angle from the horizontal. Also if you click on the middle bar it will disappear, thing that will not help you see things clearer. The disappearance of the bar will make the illusion even stronger.
How it works?
Now let us reveal you the truth: the white line always, but always intersects with the yellow one even if from times to time you may not perceive it that way.
This illusion is possible due the existence of the angular displacement theory. This theory says that we have orientation detectors that tend to exaggerate acute angle and minimize obtuse angles. It has been discovered that certain cells are active only to different angles. The appearance of an extra contour of a different orientation has the effect of lateral inhibition. Lateral inhibition can be explained through the fact that neurons tend to concentrate at a certain stimulus and tend to ignore others. It increases the contrast and sharpness in visual response.
In our case the oblique lines are seen as edges in a horizontal plane. The angles that are formed by the vertical and oblique lines are interpreted as right angles. So in this theory the angels we see are misperceived.