"Don't look at the jump" is taught so that you don't make a habit of staring at the jump as you go over it. This is important because focusing on the jump you're jumping will make you look down, which in turn makes you tip forward (more and more the closer you get to the jump), unbalancing you, and making the horse look at it too. You should be looking up for your "next fence" even if there isn't one, you need to be planning your path before you jump the jump, and seeing it. That being said, I usually look at the jump until I see my distance, which is usually 2-3 strides out. At that point you're planning your path when you land.