Good video, MGM.
Its funny how people view a stitch differently. I never think of purl as going through the stitch back to front (though I understand that's what happens), or top to bottom. I think of purl as going into the stitch from the right of it, and knit as going into it from the left.
If I was to purl thinking of it as going from top to bottom I'd be lost, yet I'm sure we all end up just as in that video, but interpreting our movements in different ways.