I remember using a similar method in a high school art class in the late 80's. The class was taught using a book titled "Drawing on the Right Side of the Brain." I don't remember who wrote it. But, we would turn image upside down and would simply draw the lines and shading that we saw. The point was to remove from our minds any preconceived notions we might have about the images we were drawing. I thought it was quite effective.
It is great advice, @sextusempiricus. That's so your right brain can conquer the logical left brain and see what you are drawing as a form rather than something you "think" you know. Thank you for commenting.