If you have piriformis syndrom, the first thing we need to do is to relax the piriformis. To do this, we try to stretch it. Because of it's location (under all those layers of muscle) and it's position (attached to the outside of your thigh and to the sacrum, horizontally along either of your buttocks), it's not an easy muscle to stretch. One way people like to stretch it is called the Number 4 stretch (see below), but lots of people are doing this stretch wrong. When you allow your back to come to the floor and your pelvis to tilt, the stretch moves away from the piriformis and more into the back. Does this stretch cause harm? Not really. Does it feel good? For many people, YES! Should I do it? Well, depends on what you are hoping to accomplish. If you want to stretch the piriformis, try keeping the pelvis in a neutral position (this will have your low back off the floor most likely). I know it's hard to see because of the shadows, but you can see how my bum is off the floor on the left, and how it's on the floor on the right. When you do this, you probably won't be able to reach your leg with your arms anymore, but that's no problem. Grab a yoga strap, or a belt, or scarf, or a tea towel — whatever you have handy — and use that to pull your leg towards you. The other leg (that is rotated outward) is the leg where you are going to feel the stretch. PROTIP: if your knee on the rotated leg feels like it is being torqued in this stretch, pull those toes towards your knee to help stabilize it. Another PROTIP: you can place the heel of the leg you might have a strap around on the wall instead of letting it float in the air. Some people prefer to do this at a wall, or even sitting in a chair (like Katy Bowman does in her original piriformis post).
BUT WAIT, THERE'S MORE
WHY was your piriformis tight in the first place? What was causing it to press against the sciatic nerve? Now, I can't diagnose you from here, but what we often see is that muscles become tight and grip when they are weak. So, after you've given your bum a little stretch, you need to start thinking about how to strengthen that little muscle. One way I like to work on external rotators is with my twisted tree. Find your tree pose. Remember that tree will work your muscles more if you don't use your hand to get that leg in position. And if your foot lands on your knee on your standing leg, that's actually ok (as long as you don't feel pain in this position). Strike your pose, and start to slowly turn from side to side. It takes a lot of strength and balancing ability to do this, so if you need to keep a hand on a wall to help, do that.