A. Sit straight, even weight on both sit-bones. Push heels into the ground. Tighten seat. Pull stomach in. Shoulders are wide, Shoulder-blades together. Long back. Feel your neck tall and relaxed. Reach up from the top of your head. Lengthen arms towards the ground, reach with your fingers to the floor. Squeeze the buttocks and 'scoop' your navel in & up.
B. Pull navel in towards the spine and even more, so much so, that your back starts getting round, keep pulling the navel in and slowly with control roll down, reach with hands towards the floor, push heels into the ground. Tuck the tailbone under, pull ribs in, open the back of the ribcage and breath out.
C. After you rolled down to your level slowly roll up, every vertebrae at a time, push heels into the floor, feel even weight on your sit-bones. Squeeze the seat and slowly keep rolling up. Breath in. Articulate the back through image B into image A. However this time you are taller then when you started out. Grow into the direction of the arrows indicated. Stay grounded, sit-bones(base) to the Earth as well as lifted, your head (crown) to the Sky.