Its no secret. I put the camera on the dinner table (as I don't have a tripod yet) and set the Exposure to 30 seconds. Turned all the lights out and had my girly sit down with her hands open, told her to try and sit as still as possible. Hit the shutter button and flicked the lighter a few times in the areas I wanted to 'paint' - the slightest light gets picked up so most of the time it was just sparks from the lighter and no actual flame which stopped my girl from freaking out. Her palms must have reflected a lot of light as I hardly went near her hands >_< (you can see where my hands have been picked up at the sides of her, although I think it still looks pretty cool regardless).
Its the only picture I've taken using a lighter so I'm finnin to play around with the effect more when I get a free evening...everybody should give it a go, its great fun and incredibly easy to do whether you use a torch, lighter, children's toy, remote control (anything that gives off a small amount of light) - just don't set fire to anything or anybody...think small effort = large results!!