In the backbends, there is a tendency for most people to open their mouths, unconsciously. This may be done to ease the intensity of the stretch along the neck and back. But however, it wastes your pose somewhat since that intensity is where the beauty and strenght of the pose is. Also, the entire jawline is lined with acupressure points that are anti-aging and health providing. So, that stretch creates the acupressure massage at all those points.