A great source for Yoga Postures by Mark Giubarelli.
Ardha Jathara Parivarttanasana
Translation: Half Revolved Belly Pose
From a lying position on the mat bring the knees to the chest. Then slowly take them down to the side until the leg that is below touches the mat. If when doing this the shoulder of the extended arm raises of the mat adjust the position of the hips and leg that is on top. Leaving the shoulder hanging in the air is a good stretch but is very dangerous.
This is a really beneficial yoga posture for spinal alignment. When performed correctly the shoulders and lower back get stretched. This results in good back range of motion. This posture is usually used by back specialists and physical therapists in the beginning stages of back rehabilitation. Sometime this pose is too much of a stretch in the lower back especially with inflamed or damaged discs or with severely inflexible backs and shoulders. In any of these cases just allow the knee of the leg that is on top remain raised off the other knee rather than on top. This will allow the shoulder to get down on the mat and also increase the flexibility of the shoulder.
In this pose the deltoids, supraspinatus, infrasinatus pectoralis major and the biceps are all stretched. Lower back muscles are also stretched.
This pose is part of a Yoga for Back Pain sequence.
Vinyasa Yoga postures to transition from and to:
Urdhva Mukha Paschimottasana Eka pada Jathara Parivarttanasana Matsyasana Matsyasana Bridge On The head Setu Bandha Sirsasana Urdhva Dhanurasana Setubandhasana, Eka Pada Setubandhasana Halasana Yoganidrasana
Yoga Teacher Tips
In some cases the flexibility of students does not allow the shoulder to touch the mat and both knees to fall on top of each other simultaneously. If this is the case allow the knee that is on top to raise up further. This will keep the twist in the body while protecting the shoulder.