This is a variation on the Piriformis PNF Stretch in the Sports Massage DVD. This stretch is even more specific in isolating the piriformis by holding the ASIS to keep the hip from coming up off the table.
Pleace your clients foot opposite their other knee, then press their knee away from you while pressing down on their ASIS.
With the client in the stretch position, have your client press their knee laterally into your hand using 20% of their strength for 10 seconds. Then have them try to stretch further by bringing their knee more medial. You can assist them with this at the end of the stretch.
Some clients will feel a stitching pain in the area of the inguinal ligament when stretched this way. This is an indication they need their psoas stretched. Psoas stetches can be found on the Psoas page.
Thanks to Robert Hall for showing me this stretch.