This pose is called Balancing the Cat. You can also add movement to it by drawing the knee and opposite arm into your center and then returning to the beginning. Barb is modeling the pose for us to the left. I include this pose and it’s many variations in my Pilates and yoga classes.
It creates a strong and flexible spine, teaches balance and focus, teaches the yogi to move the body with the breath, has the potential to stretch out the chest muscles (pectoralis major and minor), the hip flexors (illiacus and psoas/ iliopsoas), tones and strengthens almost every muscle in your back and backside, and stimulates the thyroid gland with the movement of the chin into the chest, then the head arching up toward the ceiling.