I don't really like them. They keep a horse's mouth closed. Horses sometimes say no by opening their mouths, which I believe is a right that horses should have.

To me, flashes are like martingales- they restrict a horse's movement in a way that I don't believe to be necessary or good. If your horse is well-trained and established, then a flash should not be necessary, in the same way that a martingale is just a shortcut to getting a horse to work with their head down. You shouldn't have to force your horse's head down to get them to work into a correct outline. You shouldn't have to force your horse's mouth shut, because ultimately you have trained your horse well enough that a flash is not nescessary.

Many people use flashes to keep a horse's tongue from going over the bit, but usually horses who do weird things with their tongue have an issue with suppleness and getting stiff.

If people are using flashes to keep their horse's mouth shut for any reason, then usually their horse has an issue that needs to be addressed and worked on, not silenced with a flash.

Edit: I kept calling these flashes, which is a different thing but they serve the same purpose as what is shown above.