A fascinating explanation of some of the benefits of Downward Facing dog (Adho Mukha Svanasana), as seen from the perspective of acupuncture!
http://www.artofyoga.co.uk/reason-downward-facing-dog-good/#more-%27
This pose is considered to be one of the most complete poses, in that for the legs it requires the work of a standing pose, while for the arms it is an arm balance, for the front of the body it is a backbend, whereas for the back of the body it is a forward bend and, in addition, it offers the benefits of (increased circulation of blood in head) and prepares for inversions. Enjoy your practice!