Parivrtta Parsvakonasana (Revolved Side Angle Pose) is a deep standing twist and, as Patrick and I discussed not long ago, one of the most difficult postures in the Ashtanga Primary series. As Maehle writes in Ashtanga Yoga: Practice & Philosophy, this "is not really a posture for beginners."
Fortunately, there are many variations of Revolved Side Angle for one to practice in its stead. Unfortunately, I didn't take any photos of those variations, so you'll have to imagine what they might look like. There are two targets in this posture that pose the greatest challenge for most practitioners: one is keeping the outer edge of the back foot sealed to the mat. The other is bringing the palm flat against the floor outside the front foot. For many who are first learning the pose, one or the other is possible, but not both at the same time. Therefore, it can be helpful to practice different variations of the posture that will allow you to work at the pose from both ends, so to speak.
For example: If you are not able to press the palm flat on the mat, try coming onto the ball of the back foot with the legs in more of a true lunge. This will lessen the resistance of the twist caused by tension around the pelvis. Alternatively, you can practice the pose with the back knee resting on the floor and focus on the details of the twist. As a means of developing the flexibility to keep the outer edge of the rear foot down, rather than placing your palm on the floor but allowing the heel to turn up, you can press the palms at center of your chest and brace your elbow against your thigh to deepen the twist while keeping the back foot anchored. Regular practice of these variations as well as Marichyasana C will pave the way for the full expression.
Always warm up sufficiently before attempting Parivrtta Parsvakonasana or any deep twist with some Sun Salutations and standing poses. This is a very heating and cleansing posture. The rather extreme rotation of the torso aids in healthy digestion, improves circulation around the abdomen and stimulates proper function of the abdominal organs. Regular practice of this pose tones the rectus abdominus, transverse abdominus and obliques, and brings symmetry to the muscles of the back which may help to alleviate lower back pain and reduce the effect of habitual postural problems. Be sure to practice all twists for an equal duration on both sides to reap these benefits.