No kidding?!?!! That was interesting to learn and even more interesting to realize that I hadn't noticed it before. I had always had a much easier time twisting my torso to the left than to the right, which makes sense because when the pelvis is turned toward the left, it increases the range of motion in that direction. Twisting my torso to the right (to set up for Parsva Bakasana, for example) has always been more challenging. But range of motion wasn't the only place I could experience my twisted pelvis: when I stand with my toes on the same line, my right thigh is about 3/4 of an inch in front of my left thigh.
If that wasn't strange enough, let's move up the spine. The sacrum joins the pelvis at the sacroiliac joint. So while the sacrum may sometimes feel like a solid, unmoving protrusion of the pelvis, it is actually a joint that can tilt in four different directions: anterior right, anterior left, posterior right, posterior left. Because of the twist in the pelvis, my sacrum was twisted back the other way and anterior. When I stood with my feet hips-distance apart, toes lined up, and rolled my spine forward into a forward bend, my torso would get about half way to my toes and then sharply veer to the left. Talk about party tricks.
To reverse the twist in the pelvis, I started working to strengthen my right piriformis muscle (the muscle stretched in half-pigeon pose). When I went back a week later, my pelvis was still twisted, but my sacrum was now twisted anterior in the other direction. Which means that my sacrum is hyper mobile. Imagine someone with hyper mobile elbows or knees for example -- when they straighten their arm or leg all the way, it starts to bend the other direction (hyper extension; sometimes called double jointed). This, in a nut-shell, is hyper mobility, and all hyper mobility can put a lot of strain on the joint in question.
So what is the answer? Well, as yoga teaches, the answer is balance. When a very flexible person does yoga asanas, their challenge is to find their strength. For example, I've seen people with flexible backs and shoulders dumping into their armpits or shoulders during downward facing dog. Instead, they should try to create a long, straight spine, keep the ribs tucked into the body and find the strength in the core, shoulders, and legs. I've also seen students who are very flexible in the hips. During a lunge, it's easier for them to collapse into the hips, but rather, they should find the strength in the legs and lower abdomen by keeping the back thigh lifted and uddiyana bandha pulled in. For people with a hyper-mobile sacrum, like me, I have to learn to not compensate in my low back for areas in my body that are tight. For example, my shoulders are tight. When I raise my arms overhead in tadasana, I can't bring them all the way overhead while keeping my shoulders away from my ears . . . unless I arch my back and stick out my front ribs to compensate. I'm learning not to do that and like movements anymore.
I'm learning through my injury that, just as some people follow the idea of a perfect body or a perfect life, I had been following the idea of a perfect posture, one that mimicked pictures in the books. What I've learned is that my body is not ready for all versions of all postures, and that there is no such thing as one perfect posture. A better posture is one that is appropriate for the student and her or his body on that day. As the practice grows, so will the student and her or his body. As we say in yoga, practice makes practice. Practice doesn't, in yoga, make perfect.