Acupuncture has very limited application for short term pain relief. Much of the more off-the-wall evidence is highly suspect. There was a recent BBC documentary which purported to show a woman undergoing surgery whilst not under anaesthetic but receiving acupuncture. The documentary makers were later forced to admit that the woman had received three different types of tranquiliser and was as high as a kite throughout.
Much of the problem with trialling acupuncture is creating a viable placebo control. As these placebo controls have become more sophisticated (shallow needles, stage-dagger needles, etc) the effect that acupuncture has demonstrated above placebo has diminished. Other trials, that pit acupuncture against sham acupuncture (where the needles are placed at random) show no additional benefit. This means that the chi channel theory on which acupunture is founded is not true.
The
Cochrane Collaboration is probably the best starting point if you want to look at clinical evidence for acupuncture, or any treatment for that matter.
The main idea behind why acupuncture is effective for short term pain relief is the "nerve gate" theory. That by stimulating small pain receptors, you somehow shut off larger pain sensations. This is not yet fully understood.