I hate them. They're used to keep the horse's mouth shut so they are forced to accept the bit, rather than the rider trying to figure out why the horse gapes their mouth or tries to evade it.
The flash, or grackle, wraps around the horse's mouth in front of the bit to keep them from opening their mouth, moving their tongue, or otherwise trying to escape the bit or open their mouth.
It's completely unfair to the horse to force them into a contraption like that without at least trying to understand what they're trying to tell their riders. They don't gape and chew the bit to be funny, they have a reason, but unfortunately many people that resort to grackles simply think their horses are being uncooperative or fussy and do not bat an eye at what may be causing them to behave that way.
They only mask the problem, not fix it. I can't take riders with flash bands seriously.