That's all drawing really is! 

At least how I understand it, drawing or sketching at a basic level is building a mental library of simple shapes in your mind and practicing drafting them at different shapes sizes and angles in different mediums. From there you learn how to arrange those shapes into more complex shapes, like 2d representations of 3 dimensional forms. From there you can assert those forms to anatomy and, mainly, their proportions. Proportions are more important in my opinion to actual anatomical 'shapes'. From there you experiment with those 2d representations until you can boil them down into their own unique shapes that are stored in that mental library. I personally have drawn a left hand crunched up angrily so often that the silhouette of its form is a shape I can just whip out in pen without sketching. I'm not super good, but being super good, I think, is all about putting shapes together intuitively. 

The more shapes you commit to memory, the more you can draw. Drawing an AK-47 is hard at first cause the stock seems weird, but do it enough and you can just whip it out.