I'll be quite frank, Marc's video is the only place I've ever seen the thumb/pencil method explained for digital art. It's really a vestige of the traditional art and drawing from observation, which is something you would learn if you went to an art college. As a result, it's included in the videos because they're intended to be a complete art education substitute.
In reality, a better exercise in my opinion is drawing freely from observation, perhaps with the reference on a separate screen, then when you're done, overlap your drawing with your reference and see where you've missed the mark.
Doing this over and over again will increase your observation.
Finally, one last thing, there is no such thing as cheating when you're learning. Use every dirty trick you can come up with.