The technique involves two methods, moving a light around in the frame during a long exposure, similar to writing with a sparkler, on cracker night, or using a controlled light source, such as a flashlight or speedlight, on or off camera, to light a particular part of a scene.
The latter is more of a contrived and creative technique that can yield spectacular results, especially in a commercial setting. LED torches are the tool of choice here. $6.95 from Kmart, will get you a small pocket sized, but extremely bright and useful LED torch, which is also daylight balanced, so you won’t get any undue yellow shift in colour to whatever you light up. I often use colored gels / filters that go with my Canon speedlight, over the light source to further increase the dynamic feel of the image. … Read the rest