Researchers don't quite understand how acupuncture works, but there are several theories. One theory is that the introduction of the needles releases chemicals and endorphins that help manage the pain, while another theory says that the needles affect a patient's pain threshold. However it works, the Chinese belief is that the body's health is a balance of two forces, the yin and the yang.
If the forces are not in balance, as would happen with illness or accident, this must be fixed and acupuncture is able to fix this.
The acupuncturist locates various points that are believed to be responsible for certain areas of the body and by inserting the acupuncture needles into these points, balance is achieved.