The flash noseband helps to prevent the horse from opening his mouth too wide.
Originally the flash was used to attach a standing martingale on the caverson part, with the drop strap prevented the horse from opening his mouth. The action isn't as severe as the drop. Helps to keep the loosering snaffle centred in the horses mouth. Correctly fitted you should be able to slide a finger around the nose and two fingers between the flash strap and the nose.