If you are in a Yoga class where you are being directed by the teacher to stretch your Psoas muscles Before they are fluid and responsive then you risk tearing the muscle fibrils (the millions of structures that make up the complete muscle) and therefore damaging the muscles.
The very act of trying to stretch aggressively any muscle when it is still tight will be counterproductive in the long run and each time you try and restretch that muscle there will be a psychological barrier from the brain not wanting it to be stretched.
The first place to start is to Enable your Psoas muscle to be soft and fluid before you actively stretch it.
The Soaz method can do that very quickly and allow you to start your yoga Psoas stretch without fear of damage to the muscle.