/ic/ - Artwork/Critique

I finished watching Krenz, everything he says is sorta aligned with what I already know, mostly I need to work on my memory of shapes to develop shortcuts, I guess.

But can someone elaborate on what he called "sailboat theory"?
I assume it has to do with composition, establishing the ground, the vertical center of focus and some kind of leading curve, but I can't find much by googling it.

Also, he mentions this is part 1/3, did he ever release parts 2 and 3, or are there any recommended tutorials that are equally condensed?
I have been drawing for a loooooong time, I just suck at the fundamentals, so a 3h summary of all these topics was great for me to diagnose what's wrong with my art.