The following came to me via Susi Hately Aldous' I Love Anatomy: Anatomy and Asana Newsletter. Susi is a yoga teacher, anatomy expert and owner of Functional Synergy.
Susi's book, Anatomy and Asana: Preventing Yoga Injuries, is required reading in the Kripalu Yoga Teacher Training, and is one of my favorite anatomy books. It is well-written, clear, and accessible both to the professional, and the student of yoga.
An example of Susi's clear style can be read below in her excellent discription of the proper approach and mechanics neccesary to do revolved triangle pose (parivrtta trikonasana) safetly.
In order for Revolved Triangle to occur safely and smoothly, a few things need to happen.
1. There needs to be balance of the legs on the pelvis. As the twist occurs, the legs and feet can’t collapse on each other. If they do, the twist also collapses, and strain can enter into the neck, shoulder girdle, or back.
2. Although much of the initial twist is meant to occur around the base of the thoracic spine, the muscular engine of the twist is at the obliques. Because of the structure of the facet joints, the lumbar spine doesn’t have much of a bandwidth for twisting. The base of the thoracic spine however, does. The obliques, with their attachments at the ribs and the pelvis, help to gently drive that movement. Allow yourself to feel that oblique movement (or lack thereof).
3. The cervical spine (neck) twists only after the spine below has found its position. As many teachers know, students often move the neck too soon and too far. Feel the rotation in the torso below before moving the neck.
4. The shoulder girdle follows the spinal rotation. Sometimes this happens in reverse and the student gets into over-leveraging (a very easy mistake to make). Let's look at an example with your right leg in front. If your right leg is in front and you are twisting to the right, you may be inclined to have your left hand move to your right leg to . . . jusssst . . . squeeeeze a bit morrrrre . . . twwwist . . . out of the pose. In this example, the left hand and arm are driving the twist as opposed to the spine leading the twist and the shoulder girdle following with the support of the left arm and hand. It is something that I often see in students who really want "to feel SOMETHING" in this pose. Small note: I am not against leverage, but if it happens before the lower thoracic spine has been connected to the obliques and pelvis, then leverage can be downright dangerous.
I think Revolved Triangle can freak people out in a similar way to back bends. You can’t really see where you are going, and you are relying on your legs and your pelvis to be stable while the spine moves.
Nonetheless, Revolved Triangle can be truly remarkable, as many participants in the workshops can attest. If you would like to dig down a bit more, I have a Revolved Triangle teleclass ready to download. In this recording I speak more about the shoulder girdle and hip mechanics as well as the nature of the twist. Just click, here.