Hi @Alexis Stockdale, good question! I think it's a combo of having drawn a lot, and the use of perspective techniques. The foreshortening of the humerus can be figured out with perspective techniques. In this case I think I would use a circle in perspective to get the foreshortening right. The glenohumeral joint would be the center of the circle, and the lenght of the humerus the radius. I think Marshall has this technique in mind (he draws part of the circle in this example), but he also relies on his intuition. If you practice drawing boxes, cylinders and spheres a lot, you might get so comfortable with them that you can play with them freely in space. When I sketch I usually start with the intuition I've gained from practice, then if anything feels of, I can go back in and use perspective techniques to correct things. I gained much of my intuitive understanding of perspective when doing the exercises in the Figure Drawing Fundamentals course. It prepares you with the skills you need for the anatomy course. And to get a deeper understanding of perspective you can pair the figure course with Marshall's 1994 Perspective Drawing Series https://marshallart.com/SHOP/all-products/all-videos/1994-perspective-drawing-series/ Hope this answers your question :)

Attempt after following the video. I can notice the proportional mistakes in the original and I didn't identify the perspective of the humerus properly. Also I can notice the importance of identifying the direction in which spheres are facing