This is a complex question with no easy answer. When someone asks this question, they are usually looking for a mechanism in modern biomedical terms. In that regard, there have been many different proposed mechanisms invoked to explain the effects of acupuncture. These include;
a. stimulation of the nervous system
b. stimulation of the release of neurochemicals such as endorphins
c. action through non-specific counter-irritation effects
(causing pain reduces pain)
d. stimulation of trigger points known to have specific musculoskeletal effects
e. vascular effects (constriction and dilation of blood flow)
f. specific effects on structures in the brain
g. transmission of piezoelectric signals through myofascial networks
All of these mechanisms have involved fascinating research. But in truth, no single mechanism has come close to being able to explain the full range of therapeutic applications enjoyed by acupuncture and moxibustion over the last two millenia.
Furthermore, the structuring of research into the fundamental mechanisms of acupuncture is often biased by erroneous presumptions about acupuncture. For example, the entirely incorrect assumption that acupuncture is primarily a therapy for pain control led to much research based on known mechanisms of pain mediation.
The mechanism of acupuncture is a very active field of research. Much knowledge is likely to emerge in this area in the future. But knowing or not knowing ‘how’ it works is not and should not be a prerequisite for its successful use. Many therapies in medicine are routinely used without their full mechanism being understood.
In all likelihood, the system of meridians and points mapped on the human body by ancient Chinese physicians reflects all of the mechanisms listed above and more not yet thought of or investigated. The meridians and points of acupuncture represent an elegant and sophisticated map of the body-mind system that stands on its own. Deconstructing it into simplistic bits and pieces may satisfy research intrigue but may, in the end, have little to offer the clinician working with real people and their real healthcare needs.
Traditionally, the Chinese were very practical people. If something worked, they were happy to use it. How acupuncture worked was not a question that was often asked. And if it was asked, the answer would have been that it works through the mechanisms of Qi circulation and the dynamics of meridian relationships.