How to Perform the Posture
There are some crazy exercises to open the hips and regain flexibility in the pelvic girdle. Some of them involve a partner pushing or pulling legs to ever wider angles. Those just seem painful to me, and I should know, because I have tried some of them in the past.
Here is an exercise that you can do lying on your stomach that will open and relax the hip joints without having to become a contortionist.
Lie on your stomach with your hands above your head. The alignment of the body should be as in the Wuji Posture, but lying flat on the floor. Turn your head to the left so the right cheek and ear are against the floor. Breathe deeply and slowly.
Now pull your left leg up so the foot is at the height of the right knee, and the left knee is level with the hip. The left foot is below the left hip. If you cannot get the foot to knee height without experiencing some discomfort, then stop. Hold this position for a minute before straightening the left leg.
To repeat on the right side, turn your head to the right so the left cheek and ear are against the floor and then raise the right leg. Hold this position for a minute.