(NOTE: Do not attempt this posture without the close supervision of a qualified instructor, especially if you have any history of neck injury.)
Throughout your stay in this posture, be aware of the sensation in the neck. We want to maximize spinal extension without crunching or compressing the cervical vertebrae. To do this, try to envision your cervical spine in the posture. Use gentle muscular contraction to create a support structure for the neck, as if each little vertebrae is held securely in its place by 360 degrees of controlled muscular tension. Resist the temptation to lift the arms away from your chest. Keep them down and pull the shoulders back. As you feel ready to move deeper into the posture, try pressing the legs straighter. Eventually, the legs may come completely straight with the inner legs together.
To exit, exhale as you slowly roll down the the centerline of the head. Take a moment on your back to gently rock your head from side to side and release any compression in the neck before lifting your head away from the floor.