Western Medical Perspective: At the very basic level Acupuncture works because the needle is simply a foreign object in your body. This creates an immune response and the body deploys its defenses to attack the area. Bringing with it a variety of beneficial cells, immune reactions and effects. If the tissue is either inflamed or tight these effects cause the tissue to relax.
Why is the Tissue Tight? Fascia! or Connective Tissue is the reason. Its seems to be the bodies self defense mechanism or as a way to strengthen us for the Fascia to get tighter. Its why after a long break from exercise you should ease into it conservatively and not go too hard at once to prevent injury. On the other hand if you gradually build up strength and toughness overtime some people achieve an almost super human strength and toughness. For example: Kick boxers and leg checks, kung fu masters and the Iron Shirt they can cop insanely powerful blows and appear unfased, as their boxing bag is literally made of wood.
What is Fascia?
If you eat red meat. You've probably noticed the white stringy spiderweb material holding the muscle fibers together. This isn't fat, it is fascia ( connective tissue). If we needle a painful area which is either inflamed or tight. Needling fascia helps relax excessive tightness and relieve pain.
Eastern Medical Philosophy:
Acupuncture has been around for Millenia and has even been documented. Earliest documentation that refers to acupuncture procedures is The Yellow Emperor's Classic of Internal Medicine, dating from about 100 BCE.
Acupuncture system is complex and take years to fully understand however a simplistic explanation via the following Principals:
Ying and Yang: This is a fundamental law of nature and the universe it the oscillation of hot (yang) and cold (yin) which gives planet earth its life and drive to thrive. The earth is half warm (sunny side) half cool shaded side. The seasons Summer and Winter it is this change that drives life forward forcing animals and plants to move and grow in summer and contract and hibernate in winter. Male (yang), Female ( yin) all species on earth are subjected to this balance/ imbalance to drive mating and proliferation of te species. Men are naturally warmer, faster and stronger due to extra testosterone and woman are more nurturing and gentle. Without each other we cannot survive and together we are complete as we compliment each other.
From a health perspective ill health can manifest in both forms of excess yang (feeling hot/ high blood pressure) or yang deficient (feeling cold and tired). These conditions can be treated by either draining excess yang. or by warming ( adding heat) to tonify yang