Doesn't have the pressure points a flash has. To fit a flash correctly, the cavesson needs to be reasonably tight so the flash doesn't just pull down the front, it also puts more pressure on the nose as it fits just above the nostril holes. Drops are less restrictive & only have an action if the horse opens its mouth, but the flash can be felt by the horse the whole time its on.
also, a correctly fitted drop (fitted high enough, 4 fingers' width above nostril, many are fitted too low) doesn't have to be tight to be effective. they are miraculous on some horses. a drop will sit beautifully with a full cheek bit too, where the cavesson of the flash might catch the tops of the cheeks of the bit.