This technique of measured drawing is closely linked with another tutorial on angles found here: https://www.patreon.com/posts/144-drawing-with-5553528
The envelope method gives us another way of creating a framework for our contour sketch by fixing a container of angled lines around the form. It allows us to see the negative space as distinct abstract shapes. If we can draw the negative shapes and the shadow shapes accurately and in good relation to each other, then the rest of the drawing should fall into place quite easily.