Cavitation,  a pop or click sound, is often heard or felt by the
patient during the adjustment.  The current best guess is, it is
from the escape of nitrogen from the joint capsule.  Much like
the opening of a soda can, cavitation is caused by a rapid
release of air bubbles that have been under pressure within
the joint.  Cavitation occurs more often than not during an
adjustment, but it does not have to occur for the adjustment to
be effective.