What is piriformis?
Piriformis is one of the "deep six hip rotator" muscles that are located deep to the gluteus maximus.
What does it do?
Piriformis externally rotate the hip joint, and abduct the hip joint when hip is flexed. It stabilizes the pelvis while standing, controls medial rotation of the hip joint during the early phase of walking and running.
Piriformis is a descendant of the great caudofemoral elevator muscles that can still seen today extending from a reptile's femur to its tail. These muscles provide reptile with the greater thrusting force to extend its leg while running.
How to stretch it?
Lie on the floor or bed, pull your knee toward the opposite shoulder, with your opposite arm, pull your foot toward the opposite shoulder. You feel a stretch right underneath your bottom toward the tailbone.