To produce the clicking sounds, many people bend their fingers into relatively unusual positions, that their own muscles are unable to achieve, and which are not commonly experienced in everyday use. For example, bending a finger backwards away from the palm (into extension), pulling them away from the hand (distraction), or compressing a finger knuckle toward the palm (into flexion).

When a manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave solution creating a bubble or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of this gas bubble are thought to be mainly carbon dioxide.[1] The effects of this process will remain for a period of time known as the "refractory period", which can range from a few minutes to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.[2]

Mine don't do that...