OK, so you have got the physical action, very well said. This would be the way to explain Tadasana (mountain pose) and the legs do the same thing in sirsasana (headstand) and sarvangasana (shoulder- stand) Once you learn anything about yoga, the next two questions are: What purpose does this serve? and What then?

So, what purpose does this action serve? At the physical level, this serves the purpose of placing the head of the thigh bone(femur) into the center of the hip socket. Virtually all hip replacement patients have the back of the head of the femur worn off, as well the cartilaginous rim (labrum) of the hip socket. To a lessor degree, most people have this same tension and wear pattern on their hips because of tension in the six deep hip rotator muscles. The piriformis being the primary culprit. Do you remember the attachments and actions of the piriformis? The top of the femur and the anterior of the sacrum. This is one of the six muscles that externally rotates the femur and is the primary muscle that moves the sacrum bone. So, when you make the inner rotation action of the femur, with the internal rotators you are stretching the external rotators. This is something that does not happen in most pedestrian movement, recreational sports or exercise.

What then? Now we take that movement even further with turned in uttanasana (standing forward bend) and turned in dog pose (snow plow dog pose), to give the external rotators even more length and move the head of the femur into the center of the socket. This sensitizes the student to correct action in seated forward bends. Watch students sit in upavistakonasana (seated spread leg pose). Almost all will sit with the lateral part of their heel in contact with the floor and their toes pointed to the back. This is another example of the tight hip rotators and the head of the femur pulling to the back of the hip socket. In pascimottanasana (seated forward bend, west stretch) watch how, as the student bends forward, they tend to externally rotate the thighs. So you teach the correct action in the standing poses and then continue the theme to the floor for the seated poses and by the time you get to the inversions they might have the action integrated. (or it might take a few more years of practice).

About the wishbone effect. Pressing the inner thigh to the outer thigh as in prasarita padottanasana (spread leg standing forward bend). It is also called "hollowing the inner groins". This action is not so much intended to create space on the outside of the hips as it is to create space at the sacroiliac joints. Example: Stand in tadasana, keep the quadriceps and adductor muscles soft, then contract the buttock muscles to drive the tail bone forward to the pubic bone. Do you feel the external pull on the thighbones and does if feel like the hip bones are squeezing the sacrum bone at the sacroiliac joints, and does the skin at the back of the sacrum feel like it is flowing from the hips to the midline? Now, contract the quadriceps, and move the skin of the inner thighs to the back, keep these actions very strong and then squeeze the buttock muscles to drive the tailbone forward. keep the skin at the back of the sacrum moving from the midline to the hips. Feel more space at the SI joints?

So far I have answered this question strictly in the physical realm of the bio-mechanics of the thigh and pelvic bones. What then? This is setting you up for the next step of counter rotating the thigh in relation to the shin bone, similar to the "wringing the dishrag" action of the arms. And then the counter rotation of skin on the bones, And then the movement of prana vis a vis the 5 Vayus (winds)

Just a final word on language. I believe that if you used the word rotation instead of spiral you would have a clearer understanding of the actions involved and would communicate that understanding to your students in a more efficient way.

Thanks for the question, Allison

PoseMasterK