answer:I think it works the same as cracking your knuckles. The joint space, sealed by the synovial membrane, is suddenly subjected to negative pressure from mechanical tension, like pulling on a suction cup. This leads to cavitation: bubbles of gas suddenly form within the joint, causing a click or pop. The joint won’t usually crack again until the gas is reabsorbed. In the case of cracking your back or neck, it occurs at the facet joints where a vertebra makes bone-to-bone contact with neighboring vertebrae. In my personal experience, joint-cracking feels good! No wonder people happily pay money to chiropractors to crack their backs during “adjustments,” even if there is no scientifically demonstrated medical benefit. From Chirobase.org:
http://www.chirobase.org/05RB/BCC/06.html While the popping itself is quite meaningless, this influence might possibly be used to advantage in curing psychosomatic conditions—provided the patient is informed that the bone is “back-in-place” and will stay there. By the same token, however, such treatment used on the mentally unstable and nervous person can cause a great deal of harm; that is, by perpetuating a psychosomatic condition or even creating a new psychological illness.