It is a seat aid used to encourage the horse to canter. The rider should keep the outside seat bone in contact with the saddle at all times while using a very light, "sweeping" motion with the inside seat bone. It should not be a rocking motion or pushing/shoving motion. The rider needs to follow the movement of the horse.
It's also to get the correct lead. You want your horse to lead with its inside front leg to stay balanced. To get the horse to take the correct lead, you want it to have its inside shoulder slightly forward at the time you ask it for the canter. Moving your inside seat bone forward, having the inside leg at the girth, supporting with the outside rein, etc all help the horse choose the correct lead.
In honesty, the back injury I got 4+ years ago (thanks, Mia!) may limit the movement in my lower back so much that I can't do what other people do. I guess I visualize it as a movement of 4-6", and it can't be that...but my lower back is only now starting to unlock while riding. Maybe in a few years, I'll be flexible enough for it to have meaning...
Obviously, if you look at human anatomy, it would be impossible to completely isolate one side of the seat bone from the other, as they are attached by a piece of cartilage. If you sit in a chair, however, and keep one butt cheek stable, while allowing the other to move in a tiny forward and up motion, you will have an idea of what the correct movement would be like on a horse. I don't know how a back injury would affect the ability to make that movement. I would imagine, however, that most females would be much more flexible than most males in the pelvic region, generally speaking.