The excellent reference book Alternative Medicine for Dummies (which is really written for smart people like you) offers the following explanation about acupuncture:

“If you don’t buy the whole idea of invisible vital energy, explaining just how acupuncture works can be challenging….

“Some scientists think that, at least in some cases, acupuncture needling may stimulate the body to produce natural painkilling chemicals called endorphins and enkaphalins–which researchers have already isolated from the spinal fluid of patients who received acupuncture treatment. Other scientists think that acupuncture’s effects may have something to do with the ability of stimulated muscles and tissues to alter the nervous system, and in turn, affect internal organs such as the gut, bowel, and blood vessels.”