I can answer that last thing: While some people may call the terminator and the core shadow the same thing, they aren't. The terminator is just the imaginary line separating the light and dark side of a form. I like to think of it as "This is where light terminates." Everything towards the light source side of the terminator is in light and everything on the other side is shadow. The core shadow will be immediately on the shadow side of the terminator while just before the terminator is the darkest of the mid-tones. If you've ever shaded a sphere, the cast shadow is the projection of the terminator on the surface the sphere sits on. So if the light source is directly above the sphere and the light rays fall straight down and are parallel (really far away light source like the Sun), an observer will see some kind of ellipse depending on the viewing angle but the width of the cast shadow will be exactly the diameter of the sphere. The core shadow will be there darkest form shadow on the shadow side of the terminator. I think he just uses a slightly softened round brush with pressure opacity and flow.