Though not one of the 57 muscles attached to the pelvis, the psoas arguably exerts the most influence over it. It moves the legs, which move the body. In backbends, the psoas elongates, allowing the front of the body to lengthen.
The psoas is also part of our fear reflex; when we're threatened or traumatized, we curl inward. If the threat or fear isn't processed successfully, the body becomes stuck in the sympathetic nervous system. After a lifetime of this, the psoas shortens, pulling on the lumbar region of the spinal column and potentially leading to other emotional and physical problems.
Some students experience strong emotions during backbends or hip-opening postures. Don't suppress these feelings - release them (and yourself) from the tension and trauma buried within.