Brad's Iyengar Yoga Notebook
Vajrasana is a good preparation for Ustrasana because it lengthens the skin on the front of your shins, ankles, and feet so that they make maximal contact with the floor, pressing your shins maximally into the floor.   Try to get your heels in Vajrasana quite close together, touching if possible.   It may help to belt your ankles together.   Then hold Vajrasana for a good long time (maybe do some arm and shoulder work during this period) as a preparation for Ustrasana.   This pose is also good for working toward the leg actions of Gomukhasana.
Ustrasana is a back bend that tends to awaken the lower back more than the upper back, similar to Urdhva Mukha Svanasana and Salabhasana.   Kapotasana, Urdhva Dhanurasana, and Viparita Dandasana are poses that awaken the upper back more than the lower back.   Try to feel these distinctions in your practice of these asanas.
Most people like to do Ustrasana on a blanket unfolded one fold from the storage fold on their mat to lessen the pressure of the knees pressing on the floor.   You can also use a mat folded in 1/2 or 1/4 for this purpose which gives a more firm support than a blanket.
When practicing Ustrasana, you can make the pose a little easier by keeping some distance between your knees.   Have your knees at hip width at first.   Work to decrease this distance and bring your knees together as you make progress in the asana.   Kneel with your thighs vertical to the floor and use the following sequence to enter the pose:
1. Pressurize your feet before going back.
2. Engage your buttocks forward into your pelvis.
3. Begin to arch back.
4. Lift your chest, upper back, and front ribs while arching back slowly with your hands on your waist and your elbows pointing back.
5. Broaden your chest and collar bones.
6. Lift your side ribs and take your hands further down the backs of your legs.
7. Take your shoulders back as close to the floor as possible before taking your hands back to your feet.
8. Take your head back.   But do not just drop your head back; rather extend through the back of your neck.
9. Allow your fingers to find your heels.
When coming into the pose this way, most people have to let their thighs lean backward a little bit to get their hands to their feet, but then you want to take your thighs immediately back to the vertical position in which you started, perpendicular to the floor, and lift upward with your torso maximally.   Synchronize your thighs moving forward and your chest lifting.
When you are mature in this asana, you can also enter it by pressing up into it from Virasana, though that method is slightly more difficult than arching back into it from kneeling position.   Also, work toward going into the pose taking both hands simultaneously to your feet rather than one hand at a time, though this is difficult for beginners.   In Ustrasana, remember to keep your eyes open as closed eyes may bring on dizziness.
Actions of the legs and feet
Keep your thighs perpendicular to the floor.   Do not let them move forward or backward from the perpendicular line.   One way to practice this is to do Ustrasana facing a wall with your thighs pressing into the wall (knees touching the wall).   Begin with your thighs on the wall and keep them there as you go back into the pose.   Or if you have to take your thighs away from the wall to move back into the pose, when your hands are on your feet, then bring your thighs back forward to touch the wall.
Try to be more on your inner knees than on the middle or your outer knees.   Press your inner knees down and lift your inner thighs.   Also lift the backs of your thighs toward your buttocks.   This action is important for all backbends.   In all backbends, lift from the bottom of your buttocks, not from your lumbar spine or your navel, and raise your hamstrings.
As in all backbends, roll the fronts of your thighs inward, so as to keep your thighs and knees from turning outward with the tightening of your buttocks.   Even before going back into the pose, hug your thigh and hamstring muscles to your thigh bones and draw your muscles upward on all side of your thighs.   Lengthen both sides of your feet back equally and press your feet and shinbones strongly into the floor.   Maintain these actions throughout the pose.   Pressing your shins into the floor is an action that helps you to lift your spine more.   Especially remember to keep your feet as in Virasana.   Press into the little toe sides of your feet and turn your inner heels toward your outer heels.   Stretching through your inner heels helps you to turn your thighs inward.
Actions of the torso, hips, and pelvis
In back bends, the back of your body is active and supports the front of your body, which basically just relaxes.
For backbends in general, try to engage three aspects in this order -- (1) vertical expansion, (2) horizontal expansion of the collar bones and ribs, and (3) moving backward.   Ustrasana is an excellent pose in which to practice this work.   Remember your vertical expansion first, then horizontal expansion, back ribs lifting, and collarbones to the sides.   These things occur both before and during your arch backward.
Do not push your navel directly forward in the pose.   This does not respect the space of your low back.   Instead, lift your navel upward toward your chest.   Also tuck your tailbone underneath and draw your pubic bone upward toward your chest.   Lengthen the front of your torso.
Draw your tailbone forward and lift your back ribs toward the ceiling as much as possible.   Keep lifting your sternum toward the ceiling and rotating it toward your chin.   Move your lower back forward and upward while lifting your upper back upward.   Keep your upper back lifting so you donít compress your lumbar spine.
Feel a buoyancy in your lungs -- keep lifting your chest throughout the pose.   Lift your armpit chest and the top of your chest more and more.   Lift the top of your sternum bone.
Actions of the hands, arms, and shoulders
Draw your shoulder blades into and down your back to help expand your chest.   Lift your inner arms up strongly to help support and lift your upper chest.
When your hands are on your feet, rotate your upper arms outward so that your elbow creases face to the sides.   Press your palms down firmly onto the soles of your feet.   Try to place your full palm surface onto your full sole if possible.
When coming out of backbends in general, you want to inhale unless you have low blood pressure in which case you want to exhale.   In between repetitions of Ustrasana, it is good to rest by sitting on your heels (in Vajrasana) to keep the memory of keeping your shins pressing down on the floor.
Backbends are controlled spinal hyperextensions.   Child's pose (Adho Mukha Virasana) is a useful pose after any backbend or forward bend to release any lower back tension you may have developed.   Since this tension can be subtle, it is good to make it a rule always to do one or two counterposes after backbending or prolonged forward bending.