In another question someone linked to this video where the lady ties a knot in a seemingly impossible manner. What I don't understand is how the end sticking out beyond her right hand gets longer as she pulls through (seems that if it's to change it should get shorter). This suggests to me that there is some footage cut out.
Sleight of hand! I learned to do this by watching the video - there is a moment when you let go of the end of the rope and grab another part. Your observation is telling you which end to let go of, when to let go, and where to grab.