There is a great article in About.com about the psoas being more than a hip flexor. I have had the great fortune to study with Eric Franklin, Amy Matthews, BodyMind Centering and Nicola Weiner, PT, to get me to feel this. In Tom Myers book, he has a great drawing of the psoas and quadratus. The psoas looks like a column within the body. I image the column in front of the spine. Instead of pulling the ribs down to solve hyperextension of the ribs, I give more support of the psoas against the spine like a flying buttress.
Excellent sources to read are Liz Koch and Eric Franklin.