Welcome to the forum!!
The grackle is also known as a crossed, figure-8 or Mexican noseband.
I've commonly heard just the figure-8 reference but it does have other names.
A grackle noseband was designed to hinder a horse from crossing its jaw, gaping a mouth while ridden while it applies a more even pressure across the facial bones. They say the grackle offers ease of breathing for the equine athlete.
A flash noseband is just a loop of leather commonly that is attached to a conventional caveson that then goes around the mouth to accomplish much the same thing as the grackle.
The problem with using a attached flash is it takes the proper placement of the noseband and moves it to a position that can compromise the horses sensitive nasal bones and cavity.
Without the re-positioning of the noseband though using the flash attachment could actually possibly hurt the horse who wears a bit with the sharpness of angle and pressure points it would create.
Common I've seen when people use the noseband with flash attachment that the entire noseband is now tilted/pulled down into a wrong positioning..
You can also buy a separate buckle-on version to make a conventional caveson become a flash noseband.
The grackle, figure-8 or what ever you want to call this has indeed great use and purpose when applied correctly and for the right reasons.
It also can be applied or used for the wrong reason.
Today many pieces of training equipment are used, bought and applied because someone saw it on so-and-so horse and liked the look....please don't go there for that reason.
Training equipment is purposeful, using different than is common, everyday normal should not be because of fad or wanting to look like/similar to someone else's horse....
Use training equipment because the horse needs it while learning to overcome a problem area in their development is
The grackle noseband I was told was named after a famous racehorse Grakle, so also told the way commonly spelt is wrong too if you are paying homage to the racehorse...
Now why a noseband is named after him...have no idea!
For that matter, not positive the racehorse named after is legit or just a line someone told me.
I did find you a listing of different nosebands, a brief description of them and if they have other names associated to them by Wikipeida. https://en.wikipedia.org/wiki/Noseband