Glock switch
A Glock switch is a small device that can be attached to the rear of the slide of a Glock handgun, changing the semi-automatic pistol into a machine pistol capable of fully automatic fire. Glock does not manufacture switches to be attached to their firearms, nor do they design their firearms with the intent of having machine gun conversion devices installed. They are aftermarket parts manufactured and sold illegally through the black market or made at home using a 3D printer.
As a type of auto sear, it functions by applying force to the trigger bar to prevent it from limiting fire to one round of ammunition per trigger pull. This device by itself, regardless of whether it is installed on a slide or not, is classified by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) as a machine gun, making possession of the device illegal in the United States under most circumstances.
For reference, Glock's law enforcement & military Model 18 machine pistol has a built-in select fire function. This allows the pistol to fire at approximately 1,200 rounds per minute, meaning it could empty a standard capacity 17 or extended 33 round magazine in 0.85 or 1.65 seconds respectively.