In yoga, we have a love affair with internally rotating the legs. Down dog: turn the legs in. Standing forward fold (uttanasana): turn the legs in. The list goes on. It goes to the point of where internal rotation of the legs is seen as a panacea–a cure-all for aches, pains, misalignments and imbalances– and external rotation of the legs, as a consequence, is seen at best as mostly undesirable.
The reality, as always, is quite more nuanced than that. Simply mechanically turning the legs in, which in my opinion has little to do with the muscles of your inner thighs (also known as the adductors), produces a bunch of unintended consequences.
Further, simply turning the legs in like that, from the outside if you will, strains and compresses the femoral triangle–home of the femoral nerve, femoral artery, femoral vein, femoral ring/canal (lymph nodes, drainage). The femoral triangle is basically the armpit of the thigh. Its boundaries are: the inguinal ligament (your hip crease from hip point to pubic bone), adductor longus (one of inner thigh muscles), and the upper portion of the sartorius (from the hip point to where it meets the adductor longus on its way down and in to the inner knee).
Some of you will say that it’s not about turning the legs in (and it definitely isn’t), but about taking the inner thighs back. I don’t disagree. But I would venture to say that when most people hear the instruction to take the inner thighs back, they will turn their legs in from the outside like I described above. In other words, they will simply internally rotate their legs.
It’s quite a bit more subtle than that. The inner thighs activate from the ground up, more specifically from the lift of the inner arches of the feet via the tibialis posterior muscle.
So, for example in down dog or uttanasana, with the feet on straight and about hip width apart, draw from the balls of the feet back towards the high point of the arch right in front of the heel and lift that energy up into the inner thighs and the core of the pelvis. Then, ground down through the outer heels without losing the lift of the arches and resist the inner thighs away from each other without changing the position of your feet/heels. The pit of your belly will probably hollow out and lift by itself, your low back will feel broad and supported, and your hips won’t grip.
Excerpts from Why Internal Rotation of the Legs is Misunderstood by Anna Karkovska McGlew