Icelandic horses came over with the first settlers. They know a thing or two. Here in Eiðar, you can see the technique for getting at the good stuff: you can strrrrrrretch that border, but you never, technically, really, for honest and for true, cross the line.
Also in Eiðar, you can see just how flexible this rule is below. The Icelander on the left has one hoof back behind the line, and the one on the right has the line running right down his midline.