The majority believes "function of the trigger" refers to the action of the gun itself - whether one movement of the trigger releases more than one bullet.
The minority argued that if you are exerting constant pressure backwards, that constitutes "a single function of the trigger" and if the gun does a bunch of bouncing around or trickery to turn that into multiple shots it becomes a machine gun.
Both arguments are pretty pedantic, but I am going to side with the majority on this one - as both sides point out, anyone is capable of bump firing guns or shooting rapidly without the aid of novelty devices. And the minority argument that any gun can be a machine gun if you fire it rapidly enough seems like a blank check for the legal system.
If congress wants to ban bump stocks, they need to actually pass a bill and not let the ATF rely on a loose legal interpretation.