Beginner: Beginners benefit from downward facing dog as it stretches the muscles of the upper back, shoulders, hamstrings, calves, right down to the arches of your feet! Don't worry if you cannot straighten your legs in this pose, it will come with time. For now, focus on keeping the back super flat and your knees deeply bent. Begin to push the legs straight while the back stays flat, opening into the tight hamstring muscles. If you have wrist problems, be very mindful of any pain you feel in this posture.
Advanced: To deepen in downward facing dog, the advanced practitioner can focus on sinking into the heels and lifting the tail bone. Keep your arms super-straight and energize the palms of the hands and fingers.