Stare at the up and down motion of the foot and picture it going the other way, as a whole image your brain is seeing what it expects, as a simple up down motion it's easier to break down.
Another thing similar is drawing a 3

box, take this logo for example, if you look at the this, you can shift the perspective just by concentrating on different corners:
http://tinyurl.com/o2lx8zr