Skin practice by algenpfleger CA - what's the rules?

[Maidith]

There are no "rules". He simply tried to find out what makes things look realistic. How it works in Linda's paintings. How does light make the form 3D, and how would it look with such and such color scheme? etc.

But that's just my guess. If you really want to know his thoughts behind those exercises, why don't you go ask him directly, he doesn't bite :)