„Closed-Chain-Movement“: Instead of the insertion of the muscle, its origin moves. This is important to know!
Place the hands with the palms facing down on the knees in Chin Mudra. Then, while holding onto the knees, attempt to draw the hands back towards the torso. This engages the latissimus dorsi. The hands are constrained, so the force of contracting the latissimus is transmitted to its origin along the midline of the back.The result is what is known in kinesiology as a “closed chain” movement, whereby the origin of the muscle moves (instead of the insertion). Activating the lats in this manner lifts the spine and expands the chest forward. If you tend to hyperextend the lumbar, then engage the abdominals to counteract this. Note the effect.
Read the full article: Sitting Up Straight and Expanding the Chest Forward in Sukhasana.