This article from My Republica has a great explanation of how acupuncture works. The following is an important clarification I would make about the article…
The two sides of our body energy are, in fact, called yin and yang–not “ying” and yang as the article calls them. Furthermore, the description of them as yin[g] being “negative” and yang being “positive” energy is incomplete as most relate positive and negative to being “good” or “bad”, however, the positive and negative used here is likely defined in an electrical or magnetic functionality and is neither “bad” or “good” but instead two sides of the same circuit. Where the circuit is not complete without the other. This is an important concept when studying East Asian Medicine and Acupuncture. You can find the full article here: Understanding Acupuncture