It's called an "auto-burst". I've used them in a couple of guns. It is not very accurate as you have to let the weapon move in recoil to allow this device to operate. It utilizes a spring that presses on your trigger finger (which also mut be held loosely) and increases the frequency of the pulses on your finger to overcome the disconnector in you firing mechanism. There also used to be a crank type mechanism that clamped onto the trigger guard and as you cranked it (ala gatling gun ) pressed repeatedly against the trigger. This was more accurate to shoot as you could aim the firearm and crank it. It was however very cumpersome to use.