Are the red stripes on the barber’s pole moving up and down? Or back and forth? You could guess, and you might be right. But it really could be either and you wouldn’t know it.
Use the Show button, and see what is really going on behind the scenes. Were you right?
Now change the direction of the lines with the Horz/Vert button, and Hide the mask again.
The lines appear the same no matter which way the scene is really moving behind the mask. This means the mask must be causing the illusion.
Think of the mask as a kind of lens, through which we are allowed to see only part of what is happening behind the scenes.