I was practicing mark making for an upside-down ATC challenge. The challenge was to place your reference upside down while drawing.
Betty Edwards also suggests this technique in the book “Drawing on the Right Side of the Brain”. It forces your brain to see your reference in a new way. Which forces you to see the shapes verses quickly interpreting the shapes into left-brain symbols.
Reference from my Faces pinboard.
This was drawn with a Micron .005 black pen and then colored with Inktense pencils.