It's called light graffiti photography. You set the camera for a long term exposure, and move your arm / whatever body part you want with the light attached. The light is so bright, compared to everything else, that it leaves a mark, even if you moved it for a second. If you stay in the same spot while this is happening, whatever body parts you don't move will (hopefully) be clear.

Thanks! appreciate it.

No problem! I experiment with this myself, and the only good thing I have gotten is an old teddy-bear we have smoking a cigarette looking out into the rain. It's very difficult if you are trying to highlight an object / write something and you are the only person.

i think you have to set up a long exposure time on the camera. i don't know how it works but it does haha