Also known as the riser, the yoke is the v-shaped section at the back of jeans. It gives the denim a curved seat, and the deeper the V of the yoke, the greater the curve.
There are different types of yokes, ranging from straight to v shape to no yoke at all. Designers use yokes to create different illusions, from enhancing a small backside to de-emphasizing a larger one.
Diagram of the various yokes: