Brad's Iyengar Yoga Notebook
This is initially one of the more challenging of the standing poses.   Parivrtta Trikonasana is a forward bend, a twist, and a balance.   In this pose, your spine is in extension as well as rotation.   All of the standing asanas including Uttanasana and Adho Mukha Svanasana help you to prepare for this pose.
If you find Parivrtta Trikonasana too difficult initially, you should modify it and use props as much as necessary to allow you to feel the spirit of the pose, that is, the spinal twist.   For instance, take a much shorter step between your feet than the asana usually calls for.   Use a block inside and close to your front foot for your lower hand to press on.   Do not try to place your lower hand outside your front foot at first.
Go ahead and allow the heel of your rear foot to come up off the floor and bend your front knee as much as you need to in order to be able to lengthen your torso and get the feeling of the spinal twist.   Just allow your legs to do what they need to do to get your torso into good position.   Do not be attached to keeping your rear heel on the floor if you're not ready for that yet.   This sort of ability just comes with time and practice in the pose   Once your torso is in good position, then you can work backward to add in the leg position by working first to straighten your front leg, and then slowly grounding your rear heel.
When you are at the stage that you can either keep your rear foot flat on the floor or have better alignment in your spine and torso, work one way one day and the other way the next.   Train all aspects of the pose as you are able.   Through these methods you can approach the pose gradually if it seems too difficult at first.   Remember, the spirit of a pose is always more important than your ego, so use as many props or simplifications or adjustments as you need to experience the essence of the asana, or the essence of different aspects of the asana, while you are training your body to move into the complete classical pose.
Actions of the legs and feet
As in the other wide-legged, asymmetrical standing poses, you want your feet to be aligned so that a line drawn back from your front heel intersects the middle of the arch of your rear foot.   You will need to turn your back foot inward more in this asana than in Utthita Trikonasana.   If it is difficult for you to balance in this position, you can move your back foot forward off the line between your feet (which will also make positioning your torso into the asana easier at first).
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.   You will need to turn your rear foot inward a lot in this pose, but exactly how much you turn it inward will depend upon the inward rotation of your rear thigh.   Through experience, you will learn to feel what the proper angle is for your rear foot that will allow you to keep it flat on the floor.
Have the feeling in your rear foot of moving your inner ankle around the front of your foot toward your outer ankle (this outward action is counteracting the inward rotation of your rear thigh).   This has two effects -- to press the outer edge of your rear foot down, and also to help lift the inner arch of your rear foot.
As in all standing poses, it is especially important to engage your leg muscles strongly to your bones, hugging in from all sides.   Tighten the skin and muscles (quadriceps) of both your thighs to lift your kneecaps toward your groins.   Take your thigh bones deep into the backs of your legs.   Lift the skin on the backs of your thighs and hamstring muscles toward your buttocks.   Raise the inside and outside of your knees equally.   Although all of your leg muscles are drawing in and upward, have the feeling of lengthening and extending your bones outward.   Also adduct your legs (feel as though you are drawing them together) to keep your hips compacting which is an important action for the revolving in this pose.
Revolve your rear leg inward as much as possible (just as in Parsvottanasana) while still keeping a strong backward action in the back of that leg.   Draw you rear thigh bone deeply into the back of your leg, drawing back through the inner aspect of your rear thigh more than through the outer aspect to assist the inward rotation of your rear leg.   Your rear leg should have a twisting or spiral feeling to it, rotating inward throughout the pose, so take the skin of your rear inner thigh and inner knee back toward the wall behind you.   Your rear leg in this pose is in Adho Mukha Svanasana leg mode, so your shin bone is drawing back into your calf, whereas your front leg is in Uttanasana or Utthita Trikonasana mode, with your upper shin bone drawing forward to prevent knee hyper-extension.
Often when we feel unstable in this pose it is due to not pressing down the big toe mound on the inner edge of the forward foot.   The rear leg is also a foundation of balance in this pose.   If your rear leg is shaky you will tend to lose your balance.
Actions of the torso, hips, and pelvis
There is the tendency in Parivrtta Trikonasana before going into the pose (when you are turned toward the side) not to rotate the rear hip strongly enough forward to bring it in line with the front hip (which will become the "upper" hip).   Establish a strong forward action in your rear hip, combined with a rearward turning of your front hip and buttocks before taking your torso down into the pose (just as you do in Virabhadrasana I or Parsvottanasana).   Do not allow your belly to poke forward at this point.   Move your rear buttock bone away from your tailbone to assist your pelvis in turning forward.   Turn your hips as much as possible before taking your torso down.   Try raising your rear heel off the floor and turning your hips even more and then putting your heel back down without losing what you have gained in your hips turning.   Keep moving your front ("upper") hip strongly back throughout the pose.
The entry into this pose is similar to Parsvottanasana.   The difference is you turn from your abdomen (first), then your ribcage, and then your shoulders.   The pelvis should be level to start with, but in the pose, your rear leg hip must drop down just a little to enable the full twist.   Turn your torso on the way down into the pose.   Do not wait until you get down to turn your torso.
Bring your chest over your forward leg and twist your torso to have both your shoulders in line with your front leg as much as possible, shoulders perpendicular to the floor.   Your spine should be directly over the line between your feet, including your tailbone.   Your head should be approximately over your front foot.   The tendency is for the spine to move off the main axis.   Move your lower (rear) kidney area away from your rear thigh to lengthen your torso and then also draw your lower kidney more toward the center line (i.e. toward the rear wall).   Do not allow this action to round your back, though.   Have the feeling of concaving your thoracic spine, although that is more of a feeling than an action.
As in Trikonasana, rotate your underside ribs forward and take your upper ribs back.   Keep turning your underside ribs upward toward the ceiling to aid in the twisting of your spine, digging them deeper into your body and turning them under and upward more and more.   Keep recharging this revolving action.   Feel that there is a spiral all the way up from your grounded rear heel, up through your rear leg, and into your torso.   Keep your upper kidney full while your lower kidney draws into your body.
Elongate your spine out of your hips, moving your tailbone and the crown of your head away from each other.   Move your kidneys and chest away from your pelvis.   Lengthening out of your pelvis from your hips to your armpits will help you in the rotation of your torso.   Strive to feel the twisting in each segment of your spine from your tailbone up all the way through the spine in your neck and the crown of your head.   This applies to any twist.
Just as the outside of the femur of your upper leg is drawn downward toward the floor in Supta Padangusthasana, drawing into your hip socket, similarly the outside of the femur of your front leg is drawn upward into the hip socket in this pose.   Draw the outer edge of your front hip back away from your front foot and also move it inward toward the center line of your torso in order to bring your hips onto the main axis of your pose.
Your rear (lower) hip must drop down slightly to enable the rotation for this pose -- the hips are not level as in Parsvottanasana, but if your rear (lower) hip moves forward rather than both hips staying equidistance from your shoulders, it means you have rotated your pelvis too much, meaning you have dropped your rear leg hip too much toward the floor.   In the correct amount of hip rotation, you should feel the twist more in your upper back and less in your lower back.   It is the two indentations on the skin superficial to your SI joints (the dimples of Venus) that should be equidistance from your shoulders.
If you feel a gripping in your abdomen, there is probably not enough space created between your front thigh and your trunk.   Move your outer front (upper) thigh farther back, open your chest, and move your chest forward.   Hit your outer femur bone back as much as possible so that space is created.   Try to get the maximum distance between your head and your front (upper) hip -- keep pulling your front hip back and back.   Almost everyone needs to draw their rear hip more toward the center line and back (toward what actually is the "side" wall of the studio) behind you (i.e. in the direction of your heels).
Draw your front (upper) hip back toward your back leg and move your sternum away in the opposite direction to lengthen your spine and front torso.   Continue to draw the tops of your thighbones back as you draw your sternum more forward.   Pull the roots of both thighs (your groins) back toward your rear heel.
As in almost all poses, draw your shoulder blades into and down your back to assist with opening your chest.   Press the skin of your back in toward your sternum.   As you inhale, lengthen your torso more out of your pelvis.   As you exhale turn your torso more toward the ceiling.   Keep repeating these actions.   Keep cutting your lower side kidney inward and raising your upper hand strongly toward the ceiling.
Actions of the hands, arms, and shoulders
If flexibility limits you in this asana, keep your lower hand on the inside of your front foot rather than trying to reach outside your foot right away.   When you are able to get your hand outside your foot, you will be able to use the leverage of your arm against the outside of your front leg to help rotate your torso and clarify the twist in your spine.
Broaden and separate your shoulders away from each other.   Press your shoulder blades forward into your chest to help you open your chest.   Separate your arms apart maximally to help open and broaden your chest.   Use your upper arm in particular to help lift and open your chest.   Draw your triceps into your armpits, especially the lower one, but even as your muscles are drawing inward, maintain the feeling of your bones extending outward.   Externally rotate both of your arms.
As with many asanas, a wall can be a helpful aid in Parivrtta Trikonasana.   Take your upper hand palm up the wall (so you are facing the wall in the pose) and use it to press the wall to help with your torso twist, or experiment with doing the pose with your back against the wall to help press your torso more into the plane of your legs.
One way of using a partner to help you in the pose is to do the pose with your rear hip on the wall and your upper arm going up the wall (with its dorsal aspect on the wall) so you are facing the center of the room, your back to the wall.   Your partner pushes your upper shoulder back into the wall and your front hip back away from your front foot, that is the partner pushes in the direction that your front heel is pointing.   This work can make a big difference for you in your ability to be aligned in the classical pose.
Another way to work is using the wall ropes.   Stand in Tadasana with your back and heels to the wall with one of the upper wall ropes hanging right down the center of your spine.   Turn your left foot out about 45 degrees and step your right foot straight forward away from the wall so that your heel aligns with your left instep.   Now reach back and hold the wall rope with your right hand (which is going to be your upper hand in the final pose) high up on the wall rope as close to the wall bolt as you can.   Your torso should already be facing your right leg so slowly incline your torso over your right leg keeping a firm grip on the wall rope.   Feel how the wall rope helps you to begin to twist in the pose.   As you move further into the pose, gradually and slowly let your upper hand slide down the wall rope to allow you to take the full pose, but still keep enough tension in your upper arm and the rope so that the rope is helping your torso to twist upward and keep your upper hand pulling upward toward the ceiling.   Then repeat the other side; and feel happy there are only two sides in this pose.