Brad's Iyengar Yoga Notebook
Actions of the torso, hips, and pelvis
As you come first into preparatory Trikonasana, already feel here the actions you will do in Ardha Candrasana -- everything you need for a good Ardha Candrasana is in your preparatory Trikonasana
The foot which is to remain on the floor will have a little more of an inner arch to it than that of Virabhadrasana III which is more flat on the floor by virtue of your hips being level bringing more weight into the inner edge of that supporting foot.   Each standing pose has a different flavor in each foot.   Part of awareness in Yoga is being aware of the differences you feel in the same body part in each different pose.
You may take a slightly shorter stance for your preparatory Trikonasana as it enables you to come up with more grace and less effort.   Keep your front knee turned outward to face the toes of your front leg (which will become your supporting leg) and don't lose that either coming up or down.   Coming into Ardha Candrasana is one time when your front knee has to move beyond your front ankle momentarily to come up with ease, so your weight also moves temporarily into the ball of your front foot (even though you bear most of your weight in your anterior heel when you are in the actual pose).   As you step up into the pose, donít slide your rear foot on the floor which is bad for your rear hip (and poor form, not to mention), lift your rear foot.   Lift your rear leg into the pose with an exhalation because it's a strong action.   In general, once you're in a standing pose any further action is done on exhalation.
When you are fully in Ardha Candrasana there is a tendency to have the weight shifted inward and the hips rolling inward toward the inside of your bottom leg due to the natural fear of falling backward.   Resist this tendency by keeping your torso turned completely to the side and in the same plane as your lower leg.   You can overcome the fear of falling backward by occasionally practicing this pose with your back against a wall.   Another way to use a wall to stabilize this pose is to have the sole of your upper foot pressing into the wall.   This method will teach you the feeling of extending through your upper heel.
Establish your torso completely perpendicular to the floor.   Keep your upper hip directly over your lower hip, not falling down in front of it.   Draw your lower buttock forward and your upper hip backward to ensure this hip alignment.   Align your upper shoulder directly over your lower one.   Turn your underside ribs forward and your upper ribs back to ensure your torso is perpendicular to the floor.   Lengthen the lower side of your torso as much as the upper.   The tendency is to compress the lower side.   Draw the lower two ribs of the underside of your torso away from your groin to lengthen your underside torso.   Move your hand on the floor as far away from your foot as necessary to allow this action.   Establish extension in your side ribs and broadening in your chest.   Extend through your rear (upper) leg and take your ribs in the opposite direction.   Make maximal distance between the crown of your head and the bottom of your raised foot.
Lift your upper groin up away from your lower one, broadening your pelvis.   Tuck your lower sitting bone underneath to help open your lower groin.   Lift the outer thigh of your supporting leg.   Press your tailbone forward as in Tadasana.   Your lower sitting bone should come directly over your lower heel both when viewed in a frontal view and in a side view.   Feel how bringing your weight towards the ball of your supporting foot helps bring your lower sitting bone more over the center of your lower heel.
Don't let the weight of your torso just fall onto your lower arm like dead meat.   Keep the weight on your lower arm light.
Actions of the legs and feet
As in all standing poses, lengthen and broaden the soles of both your feet and spread the toes of your foot on the floor.   You may turn your foot on the floor outward very slightly to help with balance if you like.  The weight in this pose tends to fall to the outer side of the foot on the floor.   Resist this tendency by grounding the inner side of your base foot along with the big toe mound.   This pose is definitely done from the mound of your big toe of your standing foot.   Fix your intelligence there.   Press through the mound of your big toe (at the ball of your foot).
In Trikonasana the alignment of the front leg is based mostly on muscular effort, whereas in Ardha Candrasana the lower leg also has a component of bone or skeletal support to it, although there is a muscular support as well.   Your lower leg should be perfectly vertical when viewed from all sides.   Resist the natural tendency to have it leaning rearward slightly to assist with balance since your torso is heavier than your upper leg.   Turn the thigh of your lower leg outward as much as necessary to keep it in line with your lower foot direction.   Turn your lower knee from the inside to the outside to keep that knee facing your toes.
Establish Tadasana in both your legs.   Engage your leg muscles strongly to your leg bones on all sides.   Lift the skin on the back of your lower thigh and hamstring muscles toward your buttock.   Also draw your lower thigh up along its inner, front, and outer aspects.   Tighten your quadriceps muscles and draw your kneecaps both inward into your knees and upward toward your groins.   Have the feeling of drawing the skin of your thighs into your quadriceps, into your thighbones, and all the way into the backs of your legs strongly.
You want your rear foot to point to the side in the same direction as your torso or slightly upward but not downward.   When adjusting your upper leg, use the following sequence:
1 -- rotate your upper kneecap to the front, not higher than that
2 -- press your upper quadriceps back into your hamstrings
3 -- lengthen the back of your leg toward your heel
4 -- contract your inner thigh muscles upward toward your outer thigh
As you lift your upper leg in the pose, don't allow that action to internally rotate your lower leg.   This is a common mistake.   Make sure your lower knee stays turned outward, not just during your time in the pose, but also when coming in and coming out of the pose as well.   Often there's a tendency once a pose is completed just to fall out of the pose because we feel "we're done."
Press both feet away from you.   Lengthen especially the underside of your upper leg out through your inner heel.   Lengthen from your lower buttock down through your lower heel.
Actions of the hands and arms
You should have your lower hand fingertips on the floor, cupping your fingertips on the floor (or block if you are using one).   This cupping allows you more leverage for rotating your torso upward and balancing than if you had placed your whole palm down on the floor.   Do the same thing for Parivrtta Trikonasana if you want.    Keep your hand on the floor very light, as though it were a brush you are painting the floor with.   Your hand should be in line with your little toe (for beginners).
For more advanced students, the hand is in line with big toe.
Extend through both arms.   Extend especially through your upper middle finger.   Use your arms to broaden and expand your chest, however, donít throw your top arm back -- extend it straight up to the ceiling.
As you maintain the extending action through your arms, draw your triceps muscles toward your armpits, especially the lower one.
As in Trikonasana, try to turn your entire face all the way upward toward the ceiling to look at your upper hand, though it is difficult.   Turning your head upward helps keep your chest expanding and opening upward.   If you need to continue to look at the floor as you turn your head upward to maintain your balance, use your peripheral vision to do so and gradually try to shift your gaze upward as you solidify your balance in the pose.   Turning your face upward requires pulling your upper shoulder back.   You can use your upper hand on a wall in front of you for balance as you gain proficiency this action.
As in all balancing poses, keep your mind and body both firm.   If one shakes, so does the other.   Remember if you fall or put your foot down in this pose, you will be the first yoga student in the history of yoga to fall out of Ardha Candrasana.