The top of the "L" shape comes from the triangle, looking at it from the base. If you compare the bottom of the triangle to the top of the circle, you can see that the triangle extends over the edge of the front side of the circle. Then, since you can see the far side of the circle even over the top of the triangle, you know that the triangle doesn't cover much of the far side of the circle at all, hence that "L" shape (hope that makes sense!)