Mechanically the body is a series of arches, hinges and pulleys.
The psoas fits in the mechanical model acting as a pulley as it curves over the front rim of the pelvis on the way to the femur.
This serves to multiply the force generated when the iliopsoas contracts.
Its alignment determines whether or not it can be employed this way successfully.
The power of its psoas is based on its placement.
In and of itself the psoas will not live where it should.
There are muscles that, when properly toned, allow the psoas to be perfectly positioned.
When in this position the psoas provides powerful support for the spine all the way up to the head.
The key again is its proper placement.
In physics, pulleys are pretty simple systems design to make lifting a supporting easier through the intervention of the pulley itself.
A 500lb weight is not coming off of the floor with just my hand lifting it, but if I tie a rope to it and run that rope through a pulley attached to the ceiling, the weight will come off of the floor rather easily.
It is important to note that the psoas attaches on the back half of the inner thigh.
If you are standing with your feet parallel, not too far apart, and your thighs back and under the pelvis you should have the sense of the psoas connecting to the inner thigh which will be in the back half of the body.
Turn the feet out, tuck the pelvis under moving the thighs forward and see where the connection of the psoas moves to.
It goes to the front plane of the body.
When this happens there is no longer any tension created by the pull of psoas over the rim of the pelvis.
As a result you don't get benefits that could be derived from the support that could be offered by the pulley action of the psoas.
As a simple test see if you can feel the difference in the spine with the butt tucked under and the feet splayed out versus the sticking the butt out and turning the feet in.
There should be two very different feelings as a result.
Try to tune in to which is more supportive.