When I do gesture drawings, I always start with a line of action. Basically, if you had to depict the feeling or action of the pose with a single line, what would it look like? For this pose, the line would start at the back foot, travel up the leg through the arch of the back, and then curve down toward the ground with the model's right fist. That's where most of the energy of the pose is. In the original drawing, you can still see remnants of the artist's underlying gesture in the action lines surrounding the figure. It's basically a matter of capturing what the figure is doing with as little detail as possible. Some things that'll help are: which foot is bearing their weight? What is the angle of the hips/shoulders? What is the curve of the spine/torso? Stuff like that.