It's completely impromptu, meaning no gimmick or setup. Also, the idea to this trick is pretty basic so you can modify it to meet your needs REALLY easily. You can perform it like in their video or not. Also, this requires misdirection. It's a reasonably easy move it just requires that you get the audience to focus on the right place. I've performed this for small crowds of 4 or 5 but I wouldn't recommend more than that. Oh yeah, do not, I repeat DO NOT repeat this trick. They will be burning you mouth the whole time and there's no way you'll pull it off.
I really like this trick though. It's easy and gets GREAT reactions. I highly recommend it.
Hope this helps,
Charlie