The exact mechanisms of how acupuncture works have not been completely determined by modern science. Doctors of old were able to see and feel channels or meridians of energetic movement in the body that followed a particular direction and flow that traversed near the surface of the skin as well as sinking deeply into the internal organs. Acupuncture points are places along these pathways where the energy flow can be accessed, much like lowering a bucket into an underground well of water that flows underneath the earth. Wherever there is pain and discomfort in the body, it is described as being a block in energy flow, much like a dam or boulder affects water flow. Acupuncture serves to unbreak this blockage.
What Modern Science Has Found: Some research has found that acupuncture points show lower electrical resistance than in other locations on the skin, and these channels of energy actually generate light (articles from The Examiner Online: Does Acupuncture Work & Why Does Acupuncture Work).
Using a combination of imaging techniques and computerized tomography scans, researchers have also discovered concentrated points of micro-vascular structures that correspond to the map of acupuncture points shown in Chinese Medicine literature.
Other research has found unique structures of acupuncture points and meridians using magnetic resonance imaging, infrared imaging, LCD thermal photography, ultrasound and other CT imaging methods. Related studies using amperometric oxygen micro-sensors have shown partial oxygen pressure variations which also correspond to the meridian and acupuncture point locations in Chinese Medicine literature (Has Science Finally Confirmed the Existence of Acupuncture Points, Validating Chinese Medicine?).
The National Institutes of Health has brought forth several possible theories as to the mechanism behind acupuncture. Acupuncture releases opioid peptides, serving as an analgesic. There is evidence also that it activates the hypothalamus and pituitary gland, which could be the cause of broad systemic effects that can be witnessed. There is also evidence of altered secretion of neurotransmitters and neuro-hormones, as well as changes in regulation of blood flow, centrally and peripherally. And, research shows alterations in immune functions from acupuncture treatments (How Does Acupuncture Work).