So, what is that “pop”, like when I crack my knuckles?

According to Wikipedia, “the most likely explanation, with substantial evidence in support of it, is cavitation. 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 the solution, making a bubble, or cavity, which rapidly collapses upon itself, resulting in a “clicking” sound. This process is known as cavitation. The contents of the resultant gas bubble are thought to be mainly carbon dioxide. The effects of this process will remain for a period of time known as the refractory period, which can range from a few seconds to some hours while it is slowly resorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.”   Who would have guessed!