The joints are cushioned in a sealed sac of synovial fluid which acts as a lubricant to allow smooth movement of the joints without wearing down on each other’s surfaces.
When you exert force on those joints in a specific way, you’re pulling the joints apart which creates a vacuum like condition on that sac of synovial fluid. Eventually the fluid cavitates suddenly in a pop which releases nitrogen gas that was dissolved in the fluid. This bubble of nitrogen gas takes up the volume that was created by the vacuum. The nitrogen eventually gets slowly reabsorbed into the fluid as the force pulling the cavity apart is removed. That’s why it takes a few minutes to settle before a joint can pop again.
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