24-Second Upside Down Grayscale Face Warm-Up

Done as part of a practice session with poses of 24 seconds in length.

My current goal is: Improve my understanding of the forces of the facial skull, in terms of my facial anatomy.

Here is my latest mindblowing grayscale warm-up sketch, all done in 2 or 3 different colors. As you can plainly see, I'm really getting better and faster at rendering the tonal values and forces of the facial skull of that face, and all of its lines. So, what, and how do you think?