Play piercing is a kind of sexual play that involves piercing the skin with small hypodermic needles, usually a series of needles are used and some sort of pattern or series is designed on the skin. Play piercing happens mostly inside BDSM play spaces or relationships.
Most people report an emotional and physical rush from the experience, some consider it sexual, others describe it as spiritual, and some just say it's fun. It may or may not include explicit elements of power play, although there is an obvious aspect of consensual exchange of power as the person being pierced is putting trust and power in the hands of the piercer.
When done by someone who is trained, play piercing can be a very safe sexual activity. People can be trained at formalized workshops or mentored by experienced practitioners. There are many online resources as well as a book written by Deborah Addington called Play Piercing.