The piriformis muscle is a buttock muscle. If it is tight, it can irritate the sciatic nerve and cause referred pain down the back of the leg. The stretch may also be uncomfortable.
In case you cannot see the video, here is a still image instead -
|The right leg is the one being stretched. Start by keeping your back up straight. Once you have the leg in the figure-of-four position slowly bend forwards to apply the stretch.
Hold the stretch for 20 seconds. Repeat on the other side.