What is the popping sound during an adjustment?

The “popping” or “cracking” you hear during an adjustment is called a cavitation.  This cavitation is a result from a change in pressure inside the joint when it is gapped, releasing air bubbles.  The gapping of a joint is key during an adjustment, as this puts movement into a joint that isn’t moving appopriately.  Not all joints “pop” when they are adjusted, and “popping” a joint does not mean it was adjusted either.