The barber pole acts like a rectangular shaped lens, behind which the red lines move.
     Click on the Horiz/Vert button to change the direction of the moving lines. "That's odd...doesn't seem to make a difference..." It does make a difference! The lens controls your perception.
     Remove the lens. Click Show. See, they really do change directions! Now click Hide and change directions again. Apparent movement is forced by the rectangular shaped lens. You may Stop/Start the motion if you wish.