Imagine a disc spinning clockwise, the top edge is moving to the right, the bottom is moving to the left, and the right edge is moving down and the left edge is moving up.
By mounting that disc at an angle and changing which edge makes contact with whatever rests on it (your foot in this case), you can change the direction the disc's rotation moves it.
By mounting that disc at an angle and changing which edge makes contact with whatever rests on it (your foot in this case), you can change the direction the disc's rotation moves it.