>>5960776uh, i hate to break it to you, but learning anatomy has little to do with how to make a figure more poseable.
What Yamaguchi did was just place it in a place that gave it more range of motion and tried to hide the joints at very select spots, because it's noticeable as fuck and looks unnatural.
Basically, he cut away as much anatomy as possible and you're just looking at carefully photographed angles to make it look as pretty as possible. Of course, nothing makes the neck pretty
