I just read a really interesting piece in Gregor Maehle's book Ashtanga Yoga Practice and Philosophy about back bending. The main tenet of the piece was about what needs to be done once you have lifted into backbend.
The main thing seems to be releasing the tension on the muscles that got you into the posture and then to engage their antagonists. So around the shoulder girdle you release trapezius and deltoideous and instead engage pectoralis major (in the chest) and latissimus dorsi (down the sides of the back as if you are pulling your shoulder blades down your back).
Along the trunk you engage the abdominals, especially rectus abdomens and at a the hips release gluteus maximus by engaging psoas (in the front of the hips) and in the legs releasing the hamstring by engaging the quadriceps.
We want to make sure that we are not pinching the spinous processes of the vertebrae, so we need to lengthen the spine and back. This action is therefore performed by engaging the psoas, rectus abdomens and pectoralis major, all in the front of the torso.
We need to be protective of the lower, lumbar region of the spin as it is the softest region.
Tips to achieve a deep backbend:
1: Maintain the support of the muscles that got you into the posture then engage their antagonists.
2. Make a swiping movement with your hands towards the end of the mat (one by one I guess!) This action brings the sternum towards the wrists or beyond.
3.Walk your hands in towards your feet.
4. Engage the quadriceps as if you want to flex the hip joint. They cannot flex so they will instead work to straighten the legs.
5. Bring the hands closer to the feet (more space should have been made by the flexion of the hips)
6. Come up onto your tip-toes and lift your chest high above the shoulders. Bring your heels back down.
7. Engage your rectus abdominus and thrust your entire torso up to the ceiling. Engaging the Abs draws the spinous processes of the vertebrae apart, creating length and space within your back.
8. Ensure that your armpits, thighs and knees have a slight inward rotation.
9. Drishti is always to the nose (especially when dropping back).
10. Feel how the exhalations open the rib cage and the front of the body.
11. Drop the crown of the head and lengthen the neck.
12. Engage the latissimus dorsi to extend the arms, this will draw the shoulder blades down the back, opening the heart.
13. To stay open behind the heart, release the rhomboids and engage serratus anterior. This muscle sets the shoulder blades wide whenever weight is borne into the hands, also a key muscle in downward dog and arm balances.
14. So in all of these postures the shoulder blades need to be depressed (lats) and abducted (serratus anterior).
15. Beware of the inward rotate of the humorous (arm bone) as the inwards rotation lets the armpits flare out to the sides, an action that allows the shoulders to move up to the ears which ultimately decreases the backbend. This action needs to be counteracted by infraspinatus.
Unfortunately GM's description fails to describe how to engage the infrapinatus! If anyone knows, please feel free to advise. :)