Brad's Iyengar Yoga Notebook
Actions of the legs and feet
Your feet will need to be slightly wider apart for this pose than for Trikonasana.   The second toe of your front foot should point ninety degrees to the side.   The exception to this is if you have any kind of knee problem, then you may turn your front foot outward more so your knee can also turn out.  You want the alignment of your feet to be so that a line drawn back from your front heel intersects the middle of the arch of your rear foot.   Your forward sitting bone and knee should be directly over that line.   Your weight should be evenly distributed between both legs.
As in Tadasana, lengthen and broaden the soles of both feet.   Spread all of your toes wide and lengthen them, but do not grip the floor with them.
When moving into the pose, as your front knee is bending out over your foot, resist back with your rear thigh and knee.   Take care to bend your front leg to a right angle with your shin perpendicular to the floor and your thighbone (femur) parallel to the ground.   Your front knee should be directly over the ankle and heel of your front leg.   A common mistake is to allow the front knee to drift toward or inside the big toe.   Lengthen your inner front thigh from the groin to the knee in order to keep your knee going straight out over your foot (toward your second toe).   You may even need to feel as though you are moving your knee toward the outside of your front foot to achieve this alignment.
Lengthen the inner thigh of your forward leg from your groin toward your knee.   Draw the outer thigh of your forward leg toward your buttock.   Draw the skin and muscles of your front calf up toward your knee.   The head of your front calf muscle should pull back to counteract the rest of your leg moving forward.
Maintain a strong backward action in your rear hip, thigh, and knee so that your entire back leg is rotating away from your front one, your rear knee rotating upward toward the ceiling.   Take your rear thigh skin and bone (femur) deeply into the back of your leg, so your rear groin moves away from your front one.   Keep your rear hip, inner thigh, and knee drawing strongly back throughout the pose.   Raise your rear kneecap upward and draw it into the back of your knee.   Also lift your inner rear ankle and draw the underside of your rear leg back.   Lift with your rear leg as though you are trying to come out of the asana.   At the same time, lengthen your rear leg from your buttock all the way down through your heel.
Don't let the outer edge of your back foot come off the floor.   Press it down strongly.   Establish and maintain the Tadasana of your rear leg.
Your rear leg is straight and actively charged, drawing upward.   Your strength in this pose should be in your rear leg -- release your front one.   Your front leg and hip should release downward and relax as much as possible without collapsing.   See how much you can allow your front hip and thigh to release while still maintaining the structure of the asana.
Move your knees apart away from each other like opening a book.   You should be turning both knees and thighs outward away from each other to feel as though you are opening from your pelvis outward through your legs.   Broadening your pelvis as wide as possible will help this action and this action will help you broaden your pelvis.
If we call the wall your torso is facing the "front wall," your forward hip should be drawing toward that front wall, while your rear hip and leg and your forward knee should all be drawing toward the "back wall."
Actions of the torso, hips, and pelvis
Expand your groin areas to broaden your hips and pelvis and feel both your knees rotate away from each other.   These two feelings are part of the same action.   Keep your groins separating away from each other, your front one moving forward and your rear one moving backward.   Still, your rear hip will be turned slightly forward, not completely flushed to the side in the same plane as your legs.
Make sure you lift your frontal hip bone and move your rear leg buttock downward to level your pelvis.   A common error is to drop the frontal hip bone causing the pelvis to tilt toward the front leg.
Establish Tadasana in your torso in every way.   Keep your spine vertical; keep your armpits directly over your hip creases.   Inhale, expand and lift your chest from bottom to top.   Lengthen both sides of your torso from your hips upward toward your armpits.   Feel as though your torso is lifting upward off your hips, out of your pelvis.   Also, observe the connection between opening your chest and moving your rear thigh back.
Actions of the hands, arms, and shoulders
Much as in Adho Mukha Svanasana, your upper arms and forearms should be twisting in opposite directions from each other.   Your forearms should be spiraling inward to keep your hands facing the floor while your upper arms should be spiraling outward to help broaden your collar bones and chest.   You can practice this action by first extending your arms out, palms upward.   Feel the action in your chest and shoulders, drawing your shoulder blades down your back.   Then maintain that action in your chest, shoulders, back, and upper arms while you rotate your forearms and palms downward into the final form.
Lengthen your inner arms, while firming your outer arms and drawing your outer arm skin in toward your shoulders.   Use the lengthening of your inner arms to open and expand your chest and broaden your collar bones side to side.   Extend as much through your rear arm as your front one.   The time to extend especially through your rear arm is as you are bending your front knee to a ninety degree angle.   Charge through your rear arm as you bend your front knee and use this arm action as a counter-action to your knee moving out over your foot.   The essence of this pose is having a vertical spine and extending back strongly through your rear arm will help you achieve this alignment.   Spread your shoulders and collar bones apart maximally.
Do not allow the action of your arms to cause your shoulders to move closer to your ears.   Turn the skin on the tops of your shoulders rearward and downward along your back.   Draw your shoulder blades forward into your body (to help expand your chest) and down your back.   Move your trapezius muscles down your back and lengthen the back of your neck upward.
Once you have established all of the actions you can in the pose, you may turn your head to look out over your front arm.   Turning the head is always the last thing.