1- Internally rotate the femur by positioning yourself at an angle to the box. When you step up onto the box the femur will move into internal rotation.
2- Fix the lumbar spine by using the 12' box. This gives the same effect as using a table but eliminates the need for a table, a strap or a partner. It's an excellent self stretch.
3- Sidebend away to further stretch the spinal attachments of the psoas.
This is a simple, easy to perform stretch, that works excellent.