In this training, I explain how important (using the LTL idea) the "third side" is to creating real volume in the head of in any structure at all.
I also give you a very useful way to practice craft to build towards mastery.
In a nutshell, I show you how to approach a drawing as part of a process of discovery where each new drawing tries to improve on the last with the addition of "one new thing".
Suggested Homework:
From the reference provided, try to improve your understanding and execution over a series of simple construction of the head in the underneath perspective.
Have fun!