Sacral nutation and counternutation
The sacroiliac joint, thanks to the robust ligaments that surround it, is one of the most stable parts in the body. The small forward rotation movement of the pelvis, which can be done by calling the belly inward and allowing the lumbar vertebrae to remain in line with each other, is called “nutation”, which literally means nodding (like nodding with the head). In nutation, the sacrum leans slightly forward between the iliac crests and, in forward bend (Uttanasana), allows the lumbar spine to remain straight helping to protect the spine from hyperflexion (hunching with lumbar-dorsal vertebral arch) .
Therefore, in Uttanasana, activating the Tensor of the fascia lata (TFL) and the gluteus medius, the nutation protects the whole lumbar area.
The TFL is located on the side of the thigh and the gluteus medius is located on the side of the pelvis. The TFL originates from the anterior part of the iliac crest and is inserted on the ilio-tibial band and from there towards the outside of the anterior part of the tibia. The gluteus medius originates a little further back on the iliac crest and is inserted on the large trochanter in the upper part of the femur (thigh bone).
It is the opposite of the nutation: the base of the sacred moves up and back, and there is a decrease in the lordosis. There usually is Counternutation in Backbend.
These and other information on the yoga anatomy can be found on my Italian book “Ashtanga Yoga Prima Serie Analisi Anatomica della Pratica e Led Class” ed. Youcanprint.