Hellfire trigger is a device that allows an operator of a semi-automatic firearm equipped with it to fire at very high rates, close to those of fully automatic firearms. The operating principle is simple - when the trigger is pressed, Hellfire device disengages the trigger return spring, which is strong enough to move the operator's trigger finger. The result - by applying constant pressure on the trigger, operator can press the trigger (and as a result, fire his gun) a lot faster than normally.
It's a simple, but unsafe and inefficient device. Many users complained about the awkward firing position they had to use to shoot a Hellfire trigger-equipped firearm. It's also believed that most semi-automatic firearms are not designed to handle the stress of rapid fire, which may result in a malfunction and injury.
Hellfire trigger was deemed legal by United States' Bureau of Alcohol, Tobacco, and Firearms, and does not require a special permit.