The Sphinx Pose is a very gentle backbend, and something you can use to work your way into more intense poses like Cobra and Upward Facing Dog. Backbends are very important in yoga -- and for everyday life -- because we spend so much of our day being hunched over, shoulders rolled forward. Try this pose whenever you want to counterbalance those effects.
Lie on the mat facedown, toes pointed back with the tops of your feet on the mat. Try to get as long and lean as possible by firming up your legs, lengthening your tailbone, and reaching toes toward the back of the room. Raise the upper half of your body slightly off the mat, setting your elbows down below your shoulders with your forearms flat against the mat, palms down. Your back should be slightly arched back, with your head level and your gaze forward. Draw your lower belly away from the floor so that it's not collapsing into the mat.