Human beings, even in sleep, do not keep perfectly still. They breathe, they move their limbs, they move through space. While it is an indispensable practice to draw from a model in a fixed pose, eventually, if your art is to grow, you will need to draw figures in poses that are imagined or impossible to hold. (As a matter of fact, eventually all the figures you draw or paint will be imaginary. They will just be represented to the viewer with input or inspiration gained from the model.)
In this case it’s not possible for any model to twist that much at the waist. I drew this figure by cobbling together several different poses and unifying the results.
tan Canson Mi-Teintes 98-lb. (160gsm)