Like most pieces of tack a Flash is often abused and missunderstood
Yes a Flash is used to assist in keeping a horses mouth closed - but it should not be tight.
A Flash is also used to help with the position of the bit in a horses mouth.
The Flash noseband originally developed for an English showjumper who was called ' Flash ' . He needed to ride his horse in a drop noseband and a standing martingale - as you should not fit a standing martingale to a Drop but to a Cavesson the Falsh noseband was made to serve both purposes.
There is nothing wrong with a Flash noseband, just the idiots who fit it incorrectly - and you are always going to get people like that.
To those that say that you should not need one , they are partly correct , but not every horse is perfect and not everyone has hours and hours every day to train their horse .
I ride one of my horses in a flash noseband - I like the look of it on my mare, I do not have it tight - do not use it to clamp her mouth shut but to help with the position of the bit in her mouth.
Ingrid is happy in her mouth with a simple snaffle and a Flash, I am happy with the bit/bridle that we have so there is no problem.
Flash nose bands are not for clamping the horse's mouth shut! Anyone that tries to tell you that is why they exist or that they actually work in this regard is an idiot. If you really want to clamp a horse's mouth shut then buy an extra wide padded crank cavesson, they work very nicely.
All a tight flash does is pull the cavesson down on the horse's face and reduce that nice "clamping" action that the crank provides.
I ride in a flash because first of all it looks nice, and second of all my horse is very sensitive to bit pressure. Any extra jiggle caused by the snaffle rings is like white noise, nagging or an un-intended aid to him.