Psoas Muscles

The Pelvis is the foundation from where the practitioner begins the journey of practicing and understanding asana. An unstable pelvis is equal to an unstable spine. Also an unstable pelvis while practicing yoga can cause more damage to the body. The psoas muscles originate and attach at the lumbar region ( and insert at the femur. When tight they can pull the lumbar down, unstabilize the pelvis, and in certain cases cause disk herniations. Five to ten minutes in Constructive Rest Position is a great way to stabilize the pelvis while releasing your psoas.
http://www.getbodysmart.com/ap/muscularsystem/thighmuscles/anteriormuscles/psoasmajor/tutorial.html


To release the psoas lay on your back and begin with a soft belly breathing:  

Udiyana Bandha Pranayama:
http://www.yogajournal.com/poses/1707