That's because the art of acupuncture is somewhat related to the art of massage, whereby certain veins and muscles all relate to certain parts of your body. You back aches, a person massages your foot in a certain way and it will feel better.
Personally I haven't had it done but it does work, that I can say. It isn't so much a permanent cure to anything, more like a massage. Temporary relief till you do the same "accident" again.