The mainstay of chiropractic is spinal manipulation. This generic term is used for any kind of therapeutic movement of the spine. More precisely, it is the application of quick but strong pressure on a joint between two bones in the spine (called vertebrae). The pressure twists or rotates the joint beyond its normal range of motion and causes a sharp, cracking noise. That distinctive noise is believed to be caused by the breaking of a vacuum or the release of a bubble into the synovial fluid — the clear, thick fluid that lubricates the spinal and other joints.