An adjustment (or manipulation) of a joint may result in the release of a gas bubble between the joints, which makes a popping sound. The same thing occurs when you “crack” your knuckles. The noise is caused by the change of pressure within the joint, which results in gas bubbles being released. There are nerve endings in and around the spinal joints that are stimulated when the joint is moved or mobilized. The stimulation of the nerve endings in the spinal joints can result in muscle relaxation and pain reduction There is usually minimal, if any, discomfort involved.