Brad's Iyengar Yoga Notebook
Utthita Hasta Padangusthasana
In this asana, your lower (standing) leg, your pelvis, and your torso should all be in Tadasana (refer to the actions for that pose).   Do not let the raising of one leg deter you from establishing Tadasana in the rest of your body, particularly in your pelvis and hips.   This advice applies whether you are doing the asana with your upper leg out in front of you or to your side (Parsva Utthita Hasta Padangusthasana).
Spend more time on the foot of your standing leg than anything else at first.   Donít let the foot of your standing leg turn out, as is its tendency, keep it directly facing the wall in front of you just as in Tadasana.   Rotate your upper inner thigh of your standing leg back strongly.   The primary actions, such as pressing through the foot of your standing leg and taking the inner thigh of your standing leg back, need to be ingrained again and again.
Balancing in the center of the room is the last stage of learning this asana, not the first thing to obsess over.   Learn from a wall first, placing your upper foot on a window sill or beam, and then move to the center of the room to add the balancing aspect later when you have learned the proper alignments at the wall.
Don't bend forward toward your upper leg on the window sill if you are practicing the classical pose.   Just do Tadasana with one leg up and raise your arms overhead into Urdhva Hastasana.   It is OK to bend your torso forward toward your upper leg on the window sill as a specific exercise, but in the classical pose, your torso remains upright in Tadasana.
If you use a ledge to support your upper foot, you can stand on a block if the ledge you have available is a little too high for your degree of flexibility.
Actions of the legs and feet
Make sure you keep the toes of your lower leg pointing straight forward, not rotated out to the side (which would make the asana easier, though it would abandon the Tadasana of your lower leg and pelvis).   Similarly keep your lower leg straight with its kneecap pulled strongly upward toward your groin on that side.   This is more important than being able to straighten your upper leg.   Do not try to straighten your upper leg at the expense of bending your lower leg.   This applies whether you're doing the pose on a ledge or balancing in the center of the room.   Allow your upper knee to bend as much as necessary to maintain Tadasana in the rest of your body.   Having your upper leg straight is something that will come in time with hamstring and hip flexor flexibility.   Do not insist on it from the beginning.   The most important thing is the straightness and lift in your support leg.
As in Tadasana, your lower heel and sitting bone should form a straight line perpendicular to the floor.   There is a tendency to be leaning forward in the bottom leg, especially when you are using a ledge.   Be aware of this and insist on your bottom leg being vertical.   Broaden the sole of your upper foot from side to side and lengthen it forward to back.   Spread the toes of both of your feet.
When doing the asana balancing, if the hamstrings are stiff, there is a tendency to lean forward with the torso to try to keep the upper leg straight.   Resist this tendency with a strong Tadasana feeling in your torso.
Draw the skin and flesh of the outer side of your upper thigh in toward your hip on that side to keep that hip drawing back.   Also draw that same outer hip downward to help level your pelvis.   At the same time stretch the skin of your inner leg and back thigh (hamstrings) toward your foot on the leg that is up, so your inner heel moves forward away from your pelvis or into the wall.   This is a slightly different action than in the legs of Tadasana.
Actions of the torso, hips, and pelvis
Keep Tadasana in your torso in every respect.   Lengthen the line from your sacrum to your head.   Lift your side ribs away from your pelvis.   Lengthen the front of your torso by lifting your upper sternum away from your pubic bone.   Raise your chest strongly.   In this pose, synchronize the lifting of your leg with the lifting of your chest -- they are equally important.  Keep your chest square to the front.   Don't let your low back over-arch.
Keep your pelvis neutral at whatever height you can manage on the window sill.   The height is not as important as having your pelvis neutral.   As in Tadasana, this means centering your pelvis: front to back (so that your pelvis is neither tipping forward into the "swayback" position nor tipping back and allowing your low back to round), side to side (so that one hip is not higher than the other), and rotationally (so that your hips are square to the wall in front of you).   This squaring often means taking the hip of your supporting leg more forward so it is even with the other hip.   Also you must pull your raised leg outer hip down strongly in order to level your pelvis.   You may even need to shift your weight a little onto the inner aspect of your standing foot to do this.
As your flexibility allows, come deeper into the pose in the following steps:
(1) holding your toe with one hand
(2) holding your foot with two hands
(3) pulling your shin up even more and touching your forehead to your shin
Make sure your shoulders are square to the wall in front of you as in Tadasana.   A common mistake is to let the shoulder of the arm holding the foot draw forward.   Maintain Tadasana in your shoulders.  Do not let the arm holding your toe pull forward out of its shoulder, even if you have to keep your upper leg bent to do this.   Draw both your shoulder blades into and down your back to assist in expanding your chest and keeping your shoulders in their proper place down away from your ears.
In Parsva Utthita Hasta Padangusthasana (with your leg to the side), pay close attention to leveling your hips.   Feel your raised thigh roll outward as much as possible and your abdomen (your navel) move away from that raised thigh.