To add to your trigger education, there is also the "release trigger". That trigger does not fire when pulled, but fires when released.
Unlike the binary trigger and bump stocks, who's purpose is to
simulate full auto fire, the release trigger was made with a specific sporting purpose in mind. Turned out to be a poor idea, but it had a degree of popularity among the live bird competition shooters for a while, back in the 50s-60s.
Think of it as a shotgun "race gun" modification. The idea was that it made the shooter a little faster. The shooter would line up on the trap, pull the trigger, hold it, then call for the bird to be released. Then all they had to do was get on the bird and release the trigger to fire. A very specialized thing with a very small application and HUGE safety risks everywhere else, since it worked opposite to every other gun trigger in the world.
The trick was tried, used by some (yes there were accidents) and essentially discarded as not providing any significant speed advantage and carrying tremendous safety risks.
The Binary trigger combines the concepts of both regular trigger and release trigger, and the purpose was not for competition, or defense, or hunting, it was for entertainment. Simply for the fun of being able to shoot something that simulated full auto fire but avoided the legal definition and regulation of being an actual machine gun.
Also, to be clear in the technical sense ,
These guns had "select fire" meaning you can choose to shoot once per pull or twice per pull
They are not "select fire" meaning semi or full auto fire, a more proper term would be "select trigger function" meaning regular or binary function of the trigger. The gun always fires in the legal mode of one shot from one function of the trigger, so is not a machine gun under the definition in LAW.
Selecting binary mode means the trigger fires when pulled, and again when released, and that is two separate functions, technically.
Its a can of worms and totally dependent on the specific legal definitions, and the ATF is and has been trying to craft regulations to control the device, and still comply with the actual law. As far as I know, they haven't been successful, YET, but that isn't stopping them.
Binary triggers do pose a significant safety risk if the shooter is not well trained to use them. Just telling someone how they work isn't enough to ensure safety.
Its a range toy, made for fun, and that's all it has to be. Having fun IS a valid purpose and use, in my book. Doesn't have any practical applications and is a risk in un/under trained hands, but that's not a good enough reason to outlaw them, as I see it. Doesn't stop killjoys from trying though...