Eka Pada Sirsasana (Foot-behind-the-Head Pose)
To move toward a balanced spine with your leg tucked behind your neck, roll your right hip away from the right side of your waist. Your right buttock will shift toward the center line of the body, and you should feel more length come to the right side of your waist and spine. The stretch of the leg and hip joint will also increase, as will the pressure on your neck.
Another thing that usually happens when you put your foot behind your head is that your pelvis draws up toward your belly and your head drops down toward your chest. The result is that the front of your spine compresses and your back muscles overstretch.
To reduce this tendency, first draw your right hip away from the waist, and then press your neck back into your leg and lift your chest away from your abdomen, as if you were trying to lean back in a deck chair. As with the previous action, the intensity of the stretch in the leg and hip will increase, as will the pressure on the neck. The back muscles come more into play as you lift your chest, which prepares them for their work in the final pose. You can practice these actions while holding your leg with your hands at first, and then try them without.
Oh, M'God, Harry!
In many ways, the final pose is a lot like what we've called Supta Eka Pada Sirsasana, except that rather than "suptaing" (reclining), you are sitting up. That's a significant difference, though. Now your back will not be supported by the floor, and gravity will not aid you in bringing your leg into position. If you've done the preceding work, however, you will be prepared for these new edges.
Begin Eka Pada Sirsasana by sitting in Dandasana (Staff Pose). Press your thighs into the floor and stretch your inner calves and ankles away from you. Bend your right knee, raise your right foot from the floor, and catch the ankle and lower leg with your hands. With your right arm inside your right thigh, lift your right foot to third-eye level . Shift your right hand to your calf, and lifting the leg higher, bring your right knee back and raise your knee and calf over and onto your shoulder. Keep hold of the lower right leg with both hands. Lift your chest upward away from your abdomen and take a couple of breaths.
Now roll your outer right hip toward the floor, externally rotate your right thigh, and lift your leg so your right foot is above your head. Lean forward slightly, duck your head forward a bit, and pull your ankle behind your head. Then raise your head, pressing the back of your head into your ankle so that the weight of the leg on your shoulder is diminished. With your head and hands supporting your leg, turn your chest slightly toward the left and duck your right shoulder still farther under the leg. Keep pressing your head into your ankle; with your right thumb, roll your right calf back out of the way of your shoulder and pull your ankle behind your neck. As in the reclining preparation, you will probably need to hold your foot and leg with your hands for some time to keep your leg from slipping from behind your neck. Keep your inner ankle extended.