no it has nothing to do with the shape of the card.
he just told be how to do, i unless you are looking for it is kinda hard to figure out.
but he says this will work will pretty much any deck.
look at the distance between the number and the edge of the card both sides will be different, it is hard to notice since the difference is just barely noticable, there is maybe 2-3 mm difference
now that you know go and wow your friends, but dont tell them!