Eka Pada Sirsasana (Foot-behind-the-Head Pose)
As much as you can, try to lift your chest while you hold your leg with your hands. The pressure of the leg on your neck and back may be intense. New edges will appear, perhaps in your hamstring or your hip, or maybe in your back or your neck. Move with patience and awareness. Take your time. Keep your abdomen relaxed and breathe easy.
Eventually, when you are able to move from a very hunched-over position to a nearly upright one, lift your chin so that your neck, with the help of your back muscles, is able to hold your leg and keep it from flying over your head. Gradually reduce the support of your hands on your leg, until you can rely solely on your back and neck. At that point, let go of your leg with your hands completely and join the palms in front of your chest in namaste position. Keep your left thigh pressing into the floor and lengthen out through your inner left calf and ankle. Roll your outer right hip toward the floor and lift your chest as you did when you worked on the refinements in the reclining preparation.
At first, you probably won't be able to hold Eka Pada Sirsasana for long. Begin with 15 seconds, or whatever is possible, and build up to one minute. To come out of the pose, use your hands to lift your leg and ankle from behind your neck. Lower your right leg onto the floor next to the left leg, place your hands on the floor by your hips, and sit in Dandasana. Then perform Eka Pada Sirsasana with your left leg behind your head. After you have finished the left side and returned to Dandasana, lie on your back with your knees bent and feet flat on the floor, comfortably near your buttocks. Rest on your back for a minute or two to release any tension you may feel from Eka Pada Sirsasana. Going on to twists and then backbends will help reduce any tightness you may feel in your back and help balance your practice.
The End (Not!)
Even though you may have played your edges well and are able to do an "advanced" asana, you have hardly reached the end. Although we sometimes use the term "final pose" to describe the shape of a particular asana, there are really no final poses. New edges appear, both within Eka Pada Sirsasana and in the expansion of possibilities of other asanas. For instance, once you have become more accomplished in Eka Pada Sirsasana, there are many challenging poses you can work on that incorporate putting your leg behind your neck.
Furthermore, as subtle and difficult as playing the physical edge is when practicing Eka Pada Sirsasana (or any asana, for that matter), it is complicated by the fact that we have lots of different edges: physical, psychological, emotional, intellectual, energetic, and spiritual. You may seem to be playing your physical edge in your practice quite skillfully and yet be way off base with respect to your appropriate energetic edge. I see this in some overly ambitious students who push themselves constantly to do more difficult, demanding poses—and more and more repetitions of them. They may be achieving the physical movements of the poses, but at the same time they are irritating their nervous systems and compromising their mental and emotional equilibrium.