When a patient has a stuck or tight joint, a Chiropractic Adjustment may be required to correct the movement problem in the joint and allow it to heal. The popping sound, call a cavitation, is when pressure is released in the joint being adjusted. Its very similar to opening a can of soda (which I don’t recommend). You open the can and the pressure inside the can or bottle releases and hear the sound of it releasing. Its the same principal when it comes to a joint popping during an adjustment.
Here is a video of Dr. Hill performing an adjustment:
Is the popping sound necessary for a successful adjustment? No, its not. And more force should not be used just to get the popping sound. Research is very clear that adjustments are equally effective whether or not a cavitation happens.