TexasCHLforum.com

I may be totally off, but off the top of my head, if you are in close quarters like inside a car or next to curtains or something like that, the flash could cause injury or damage. Like inside a car, it could catch the headliner on fire, or if you had to shoot through the passenger door, it could burn your passengers face. Also, if you aren't wearing safety glasses when you shoot, it could cause eye injury.
Like I said, I may be way off, but it seems I remember my CHL instructor warning against it.

What's the reason you can't use it in competition?