Many theories exist but we don’t know it for sure. The spine consists of bones named vertebras, flexible disks between them, ligament-systems connecting the vertebras and small joints between the vertebras. In the small joints there is synovial fluid. This allows freedom of movements of the joint-surfaces. The case around the joints contains the synovial fluid. According to certain theories when the manual therapist removes the surfaces of the small joint from each other with a quick movement in order to positioning them properly again, the carbon-dioxide bubbles are bursting and the carbon-dioxide molecules are dissolving back into the synovial fluid. This burst causes cracking. With this burst the tension reduces in the joint. Another theory says as there is no air in the joint gap only fluid that’s why during removing the surfaces of the joints the “vacuum” gets the joint case into movement which causes the cracking sound.