Two cubes, seen from above, redrawn in every space between the vertex buffer and the screen. Flip between view and NDC to watch the frustum straighten into a cube.
What to notice. Nothing about the cubes changes — only the space they are quoted in. The one genuinely odd step is ÷w: in clip space the boundary is the slanted pair of lines x = ±w, and dividing by w is what straightens them into a box. Raise near and watch the near cube get cut away; notice the depth figure in the last line going almost all the way to 1 long before the far plane.