Excellent painting tutorial. In case you don't know the terms in the last description, "edge variation" means having "hard" or "soft" edges, where hard edges are crisp and good for high-detail, and soft edges are more blurry/smudged and are good for giving objects the appearance of receding in distance. In the painting above, the snake's eye uses hard edges, and its teeth and underside of the jaw use softer edges. "Occlusion shadows" are the absolute darkest parts because they are the areas where no light reaches, not even reflected or ambient light. They tend to be small and are used sparingly. Above, there's an occlusion shadow around the snake's eye. However, the shading of the eye was probably exaggerated to make it stand out more, since it's the focal point.