Your pelvis is three pieces, but they are held together very solidly and can only move as one block. So, you can only rock the inside seatbone by also rocking the outside; they are locked together. However, you can dip the outside of your pelvis lower than the inside, thus have less pressure on the inside seatbone becuse it comes up. YOu can twist your whole pevis left or right, and combine that with raising/lowering one side, you can get what feels like moving one side independently of the other. But, it is NOT independent.