Brad's Iyengar Yoga Notebook
Utthita Parsvakonasana is a combination of Trikonasana and Virabhadrasana II.   A good way to enter the pose is to begin first in Virabhadrasana II and establish the actions of that pose clearly before taking your torso down into Utthita Parsvakonasana.   When taking your torso down, make sure you don't raise your forward thigh any amount from the horizontal level you have established in Virabhadrasana II, since this is a common mistake.   Also ensure that your front knee stays in place well back over your front foot since it will have the tendency to fall inward toward the inside of your front foot if you are not keeping your consciousness there.
Actions of the legs and feet
You will need to have your feet wider apart for this pose than for Trikonasana.   The second toe of your front foot should point ninety degrees to the side.   You want your foot alignment 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 should be directly over that line.
As in Tadasana, lengthen and broaden the soles of both your feet.   Spread all of your toes wide and lengthen them, but do not grip the floor with them.
As you bend your front leg, your rear leg must resist as your front leg bends into position like pulling a string back on a heavy bow.   Your front leg bends forward, but try not to let your rear leg move (though of course, it will come down toward the floor somewhat out of necessity).   For most people, the inner rear thigh drops too much, causing their weight to come more toward the inner edge of their rear foot.
Take care to bend your front leg to a right angle with your shin perpendicular to the floor and your top thigh parallel to the ground (actually it is your thighbone [femur] that should be parallel to the ground, though this is harder to visualize).   Your front knee should be directly over the heel of your front leg.   Try to bring your weight into the front edge of your heel of your front foot.   A common mistake is to allow the front knee to drift toward or inside the big toe.   As in Trikonasana, you need a strong outward rotation of your front thigh to counteract this tendency.
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.   These two actions have been likened to a "U"-shaped direction of energy in the front leg.   Mr. Iyengar once drew a parallel between the front leg in this pose and the action of Baddhakonasana legs (inner groin lengthening, outer knee pulling into outer hip), and similarly between the rear leg in this pose and Upavistha Konasana legs (taking the femur into the back of the thigh, etc.).
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 with 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 and knee drawing strongly back throughout the pose.
Your two sitting bones should move in different directions.   The forward one should move toward your front knee (as if moving straight through your front femur); your rear one pulls upward diagonally away from the forward foot -- so they separate away from each other.
As in Trikonasana and Virabhadrasana II, the three main aspects of the rear leg in Utthita Parsvakonasana are (1) the thigh rotates outward, (2) the inner thigh hits outward toward the outer thigh, lifting the whole leg away from the floor, and (3) inner ankle lifts toward outer ankle and presses down the outer edge of the foot.   Note that actions one and two both participate in helping press your outer foot edge down onto the mat as well.
Don't let the outer edge of your back foot come off the floor.   Press it down strongly.   Each standing pose has a different flavor in each foot.   Be aware of the differences you feel in each foot in each pose.   In this asana, it is OK to have your back foot turned inward only slightly (almost keeping it perpendicular to the front foot) to facilitate pressing the blade of that foot down to the floor.   [Whereas, in Parsvottanasana for instance, we turn the back foot inward much more.]   On your rear leg cut your inner ankle upward to your outer ankle to help press the outer edge of the foot down.   Again, charge your inner rear thigh upward, and lift your inner shin.   The flatness of your rear foot on the floor (its outer edge) comes from lifting your rear leg both from its lower and upper aspect more so than it does from just lifting your rear inner ankle.   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.   Even though you are lifting your rear leg strongly, descend your front groin.   Give the back of your front thigh to the floor in spirit.   Keep your front foot light -- feel as though it is weightless.
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.
Keep your front thigh and knee pressing outward strongly into your front armpit.
Actions of the torso, hips, and pelvis
Although some people refer to Parsvakonasana as "side stretch pose," and it certainly does stretch your upper side strongly, try to elongate both sides of your torso equally.   Strive to lengthen the underside of your torso as much as the upper side.
Don't let the weight of your torso fall onto your lower arm like dead meat.   Your floating ribs should not ever rest on your front thigh in this asana (even though your lower ribs touch your thigh.).   Extend your spine actively up from your rear leg in a straight line.   Extend your torso maximally in the direction of your front leg.
As in Trikonasana, rotate your underside ribs forward and take your upper ribs back.   Rotate the skin of your lower floating ribs upward toward your upper armpit.   Have the sensation of leaning back very slightly with your torso, though that is more of a sensation that a true action.   You can use your upper hand to reach under your abdomen and pull your side abdominal flesh ("love handle") and skin across your front thigh toward the front of your body to facilitate your torso turning upward.
As you roll your torso upward toward the ceiling, rotate even your navel upward.   One way of thinking about this pose is to rotate your rear leg, thigh, your torso, and chest all upward toward the ceiling as a unit.   If your torso and chest are rolling upward well, turning your head to look upward is not such a difficult task.   If turning your head upward is difficult, try to turn your torso upward more.
Your lower ribs and torso should be convex.   Your upper ribs and torso should be concave.   Turn your rear ankle, shin, knee, hip, upper torso, upper triceps all upwards as a unit.   You can even turn your top hand toward the ceiling to get this feeling of turning maximally and then turn just its palm back toward the floor.
A pose may look excellent because someone's upper chest is twisting well toward the ceiling, but then you may observe their lower abdomen/pelvis region and see it is hardly twisting upward at all.   Strive to begin your torso twist as low down your torso as possible.   Arch your back slightly in the pose (circularize your armpit chest) and revolve your upper leg, hip, navel band, and chest toward the ceiling.   Your lower kidney should move forward into your body while your upper kidney stays full and broad.   Also descend your lower kidney toward the floor.   Feel the skin of your back moving toward your pelvis, while the skin of your front torso moves away from your pelvis toward your head.
Take the top of your sternum toward the direction of your head as you are rotating upward.   Move your thoracic spine into your front chest without pushing your navel forward.
As your front knee presses outward against your arm, also press your rear hip back in the same direction and draw your front (lower) hip forward (in the opposite direction).   All three of these structures should be in the same plane.   Think of moving your "front knee toward the back wall, front buttock (and front side of sacrum) toward the front wall, and rear thigh toward the back wall."   Moving your front knee back and front hip and sacrum forward are part of the same action.   As in Tadasana and Trikonasana, both thighs move back while your sacrum moves forward.
Expand your groin areas to broaden the front of your pelvis and feel both your knees rotate away from each other.   These two feelings are part of the same action.   Also deepen the fold of your front groin as you try to move your rear groin away from your front one.
Move your tailbone downward away from your lower spine and forward toward the front of the room.   Draw your sternum toward your head.   Extend outward through the crown of your head.
First try just looking upward at the triceps of your upper arm, then raise your chin and look up at your fingertips and feel the openness and the change this action creates in your whole body and mind.   Lift your ear to touch your arm.
Actions of the hands, arms, and shoulders
Rotate the inner crease of your lower elbow to face the direction your lower foot is pointing and press your forearm and lower leg together.
There is a strong relationship between the front knee and the lower arm in this pose.   Rotate your triceps toward your thigh and press your thigh against your triceps and then observe what that does for your shoulder.   Also make the knee-into-the-triceps action stronger than the triceps-into-the-knee action to assist in moving your front knee back.
Draw both of your shoulders back.   Especially draw your upper shoulder strongly back.   Rotate your upper collarbone back toward your shoulder blade on that side.   Let this action help to lift the center of your chest.
Lengthen the line from your rear outer ankle all the way up your upper side and out through your fingertips.   Have your upper arm essentially in line with your torso, rather than letting it slope downward too much   Keep focusing on this line and charging the lengthening action here.   The lengthening energy divides at about your upper hip.   Below your hip, your lengthening energy is moving down through your rear leg.   Above your hip, your energy is moving up through your arm.
As you twist your torso back, you twist your upper forearm forward to keep your hand facing the floor, however both of your upper arms remain in external rotation.   Extend more through the thumb of your upper hand and pull your upper triceps muscle into your armpit as you go on lengthening outward through that arm.   As in most asanas, though your limb muscles are engaging your bones and drawing inward toward your torso, maintain the feeling of extending outward through your bones.
Pay particular attention to your front knee when you are coming out of the pose so that it does not turn inward when you are not concentrating on it so much.
Since one of the difficult actions of this pose is to keep your front knee pressing back toward the wall behind you, occasionally first to take your lower hand to the floor or a block inside your front foot and press that arm into your front knee to help you get the pushing-back action of your front knee.   Once you have achieved the feeling of that action, then take your lower hand back to the outside of your front foot for the classical pose.
You can also occasionally try doing the pose with your front foot up on a block to help you feel your front hip more.   Stretch the heel of your front hand to the floor as much as possible in this variation.