Yoga is known to induce the relaxation response—to lower the activity of the sympathetic nervous system's "fight or flight" mechanism and increase the work of the more restorative parasympathetic nervous system; this characteristic could help with depression. But if that were the whole story, then rapid breathing techniques and poses that seem to rev up the sympathetic side—such as backbends and Sun Salutations—might be counterproductive to fighting stress and depression. The reality is that some yoga practices stimulate the nervous system and some are relaxing. It is the combination that is beneficial.
One of the fruits of yoga practice is the realization of interconnections. Our bodies, minds, and emotions interact in complex ways that science is only just beginning to understand. In this dense web of interconnections, nothing we do has a single effect. In Urdhva Dhanurasana (Upward Bow Pose), you bring more oxygen into the bottom of the lungs (an area that usually gets less than the upper regions), your blood pressure and heart rate rise, pressure increases in the head and neck, and you stretch the muscles and organs in the front of the body as you compress those in back, where the adrenals are located. Possibly it's the interrelated actions of this pose—along with other elements of a complete yoga practice—that create the therapeutic benefit.