This seems to me a way of notching to use when the line is far in from the outside, maybe novel, maybe underexposed, I saw emanating from the Alps off of some image material. Here I am giving it a go in the flesh. Two notches cut in the vertical way, side-by-side till half the distance in to the line leaving a diamond shape in between if we imagine a single point on the line we want to reach. With the 3rd notching that center diamond is taken out to create a single deep notch from the three.