Recent research on this subject shows that a gas (in this case nitrogen), which is normally dissolved into the surrounding tissues of the spine, rushes in to fill the partial vacuum created when the joint surfaces are slightly separated. Another example of this phenomenon would be the “pop” you hear when you take the cork out of a champagne bottle. The carbon dioxide in the champagne rushes out of the bottle when the cork is removed. This phenomenon is called a cavitation.
It is not always necessary to cause a “crack” or cavitation in order to gain the desired effect of an adjustment. The main aim is to increase movement at the specific segment of the spine to positively influence the surrounding tissues. In fact, some chiropractic adjustment techniques use little force and thus produce no sound.