The values are carefully grouped and controlled. In the left side of the jacket, for instance, he didn’t overdefine the modeling on the light side, allowing all those light tones to group together into a larger shape.
The darks are also grouped, so that the face in shadow joins at the chin with the dark shirt-front, and the knuckles link up with the blue cloth and the legs into a bigger unit.
The study is also a great example of the “windmill principle,” a tonal scheme where the figure/ground relationship includes all four basic possibilities:
1. Light against dark (knee at far right and bottom of jacket)
2. Dark against light (head)
3. Dark against dark (area around hands)
4. Light against light (shoulder and back)
All this is worth mentioning because I find the automatic tendency is to break up everything into smaller and smaller shapes and to define every edge equally. In my experience, grouping shapes and downplaying certain edges takes conscious effort.
Charles Bargue on Wikipedia
Bargue is known to atelier students for his excellent course for academic drawing that he developed with Jean-Léon Gérôme.