The flash was originally developed for show jumping riders, so they could close the mouth lower down in addition to having an appropriate noseband for a standing martingale.
An additional feature of this noseband is that it holds the bit steady in the horse's mouth, which some horses prefer. The noseband is similar to the plain cavesson in that the top part encircles the nose 1-2 inches below the cheekbone, but it also includes a second strap that runs from the cavesson, around the nose in front of the bit and under the chin groove, then coming back around to the cavesson.
This second piece is used to help keep the horse's mouth closed and to keep the horse from crossing his jaw. A flash noseband may be used with a standing martingale when the martingale is attached to the cavesson piece.
This noseband is usually seen at the lower levels of dressage, or in the dressage phase of eventing.