Greetings Yves. This topic deserves much more discussion than just the inference from a single practitioner of acupuncture like myself; however, I will do my best to give the most clear and concise explanation as possible.

To give some context, yesterday I attended the 2nd North American TCM Symposium: Acupuncture in Pain Management, in Queens NY. While there, one of the presenters, Dr. Amy Matecki, the co-founder and President of the International Center for Integrative Medicine (ICIM) and the Chief of Integrative Medicine in the Department of Medicine at Highland Hospital, AHS. She is both a MD and L.Ac (licensed acupuncturist). She noted, thought there is a great deal of research into the efficacy of acupuncture for various conditions, the exact mechanisms through which it produces it's effects, remains unknown. 

The most brief, layman's answer is: acupuncture helps treat conditions such as pain, by influencing circulation and stimulation the central nervous system to release endogenous endorphins (hormone), the body's natural pain relievers. 

You'll note in my latest blog post, I cite studies showing the effects of electro-acupuncture for the treatment of knee osteoarthritis: https://www.dmacupuncture.nyc/...

One of the issues when it comes to clinical randomized trials (CRT's), the gold standard of scientific evidence and evidence bases medicine (EBM), is that there is often a lack of funding to conduct the large scale studies that EMB considers credible. So what happens is a cherry picking of studies to support or disprove acupuncture's efficacy.

I hope this gives some clarity into a very complicated topic.
Feel free to ask me anything else.

Regards,
Damian