The sowing arm, caught mid-throw
His right arm, flung across his body to the left; the seed in the air
His right arm is swept all the way across his chest, the hand open, just past the instant of the throw — you are seeing the follow-through, the seed scattering off into the furrows. It is the single most active gesture in the picture, and Millet froze it at the most violent point of the arc, which is why the figure feels like it is moving even though it is paint.