An adjustment is a highly skilled and precise movement usually applied by hand to a joint of the body. Adjustment loosens the joint to restore proper movement and optimize function.
When a joint is adjusted, a gas bubble escapes causing the popping noise you may have heard about.
Chiropractic adjustment techniques have been researched extensively. Complications are rare and side-effects, such as temporary soreness, are usually minor. Your chiropractor is well-trained to determine if your problem will respond to chiropractic care or if you require referral to another health care provider.
(Ontario Chiropractic Association)