This article from 2011 has been edited and updated April, 2020. Find more pelvis and pelvic floor articles, exercises, and recommendations at “Our Best "Healthy Pelvis" Resources.”
The sacrum—the bone that sits at the bottom of your vertebral column—is one of the levers that moves the muscles of your pelvic floor. Whether your tailbone hurts, or you're trying to get your pelvic parts to move more, one easy adjustment you can make is to stop sitting directly on your sacrum.
Watch this video to see how to move the pelvis forward (listen for the cheering):
Some of the "pulleys" that affect the function of the pelvic floor are your piriformis muscles. Each of your piriformis muscles (here's a good picture) connects one side of your sacrum and your ilium (other pelvic bone) to your femur (thigh bone). When the piriformes don't move well, these other bones don't move well, which can ultimately affect the function of your pelvic floor.
To increase the range of motion of these muscles and thus free up the bony parts of the pelvis to move as they need to for pelvic functions (including birthing space), try a "Number Four" stretch.
Sitting (in a flat, kitchen-type chair works best), cross one leg over the opposite knee. If your hips are really tight, you might have to lean back to facilitate this movement.
But by now you might have noticed that my pelvis positioning is not good—I'm sitting on my sacrum.
So, after you get into this cross-legged position (scooting to the edge of the chair helps), untuck your pelvis as shown in the video above to involve your piriformis more. And if your hips (including your piriformes) are very tight, you can sit upon a rolled-up towel in a way that lets gravity help you "untuck."
And then your pelvis will look more like this:
Make over any tucked sitting as much as you can through the day and add the Number Four stretch multiples times through the day, working up to hold each side for at least a minute. And then, keep your ribs down while you do it. And let your head drop forward too, and see if you can feel your back expand while you breathe.