Eka Pada Sirsasana (Foot-behind-the-Head Pose)
After exploring your hamstring edge, bring your shin back up to perpendicular and change your grip to hold your outer lower leg with your right hand and your outer ankle with your left hand. Keeping your right shoulder tucked beneath your knee and thigh, externally rotate your right thigh and pull your right ankle toward your ajna chakra (your "third eye" near the center of your forehead, just above your eyebrows). Avoid pulling the little-toe side of your foot toward the floor. If you pull on the foot rather than the leg, you are apt to bend and overstretch your outer ankle. The movement of your leg should come primarily from your hip. Keep externally rotating the thigh and move your right hip away from the right waist as you bring your foot as close to your face as possible.
From the leg-over-shoulder position, begin to lift your foot toward your sahasrara chakra (at the crown of your head) until your foot is above your head. Raise your head from the floor and pull your foot behind your head until your ankle is pressing into the back of your head. When you raise your head and move your foot, take care not to grip your abdominal muscles. It is possible to cramp those muscles, which can be a not-so-sweet discomfort. If that happens, lie back and relax for a few minutes until the cramp subsides. Once your ankle is behind your head, take a few breaths. Reconnoiter. Do what you need to do to respect your edges. Don't panic. Don't get greedy. If you can go on, do so; if not, repeat the same actions on the left side.
If you're ready to go on, press your head back into your ankle until your knee is no longer pressing your shoulder and, turning your chest to the left for a moment, tuck your right shoulder still farther under the leg. Then press your calf with your right thumb so it rolls backwards out of the way of your shoulder and pull the leg down so that your lower leg just above your ankle is behind your neck. Keep your inner ankle extended so that the inner and outer ankle are balanced.
You are likely to need to hold your ankle and lower leg with your hands for a while—seconds, days, weeks—to relieve some of the pressure that the leg exerts on your neck and to keep the leg from slipping from behind your head. As your hips loosen, your hamstrings stretch, your back lengthens, and your neck strengthens over time, you'll be able to slide your leg well into the curve of your neck. Then, if you lift your chin slightly, you will be able to hold the leg with your neck and let go with your hands.
When you can do that, bring your palms together in front of your chest in namaste position and stretch your left leg straight up toward the ceiling. You'll be in what might be called Supta Eka Pada Sirsasana (Reclining Foot-behind-the-Head Pose) or, perhaps, Urdhva Mukha Eka Pada Sirsasana (Upward-Facing Foot-behind-the-Head Pose).
There are several somewhat more subtle actions that can help you refine your preparations and will allow you to be more balanced and open when you attempt the final pose. When your right leg is drawn up to bring your foot behind your head, your right hip usually comes along for the ride. This movement results in congestion in the right hip joint that inhibits its freedom; it also contracts the right side of the spine and can put strain on the vertebral discs and/or the sacroiliac joints. Your left lumbar spine and/or sacroiliac joint may then overstretch to compensate, amplifying (in a potentially problematic manner) the natural tendency toward imbalance inherent in the pose.