The purpose of this one (as with many others) is derotation of the lumbar. So obviously those with thorasic curvatures are relieved of duty with this exercise, but I know they get another one.
You must lie with your torso on a large yoga ball. Holding on to the rungs of a ladder helps keep you in place as your knee bends in with as much pressure as it can for each attempt. Arms need to be bent at 90 degrees and at shoulder height, with elbows also at shoulder height and with lats engaged, so the whole upper torso gets a bit of a work out too. No slacking off you lats!
In my case, my spine collapse is on the right, so I work my right knee. My left foot must rest off the ground, on a small stool.
To prepare, the right leg stretches back as far as possible, then the right knee bends and presses against the ball as hard as it can for 6 long seconds. Repeat 12 times, with a stretch break in between the 3 sets of 12 in total.
Lying on a ball is good, not only because it forms agains your body and lets your head face down, but you can feel your breathing against the ball on each inhale. It's also an unstable surface so some of the small stabilising muscles have to work.
I couldn't see it of course, but I was told that it was very obvious to note that the pacqet, or hump caused by the rotation of the rib cage on my left side, gets soft when I perform each knee bend. It is normally hard, as the ribs stretch around in a form not intended. Someone said it was even noticable by just looking at it, which was preferable to everyone poking my back.
But I didn't really mind the poking. I promised myself I would open myself up to the entire experience in case it was of use to anyone else, mind, body and soul - and that means letting everyone poke me and telling me what they see and what they feel. So just in case they were right and it is possible to see it working, I present two photos:
This is surprisingly not that hard to duplicate on the road. The best is if I am staying somewhere with a single bed, but is can work okay with a double bed too. I drape myself over the bed and hang on to one side or one corner, while my right knee presses against the bed side. I can usually find a small stool or my suitcase to rest my left foot off the floor and at the right angle.
*note that this exercise was prescribed to me for my particular scoliosis, and it might not be appropriate for others.