The Lightning Link is one of those unique solutions where someone happened to look at the operation of the AR–15 and how the various parts of the operating systems work together to find an easy way to convert a semi–automatic firearm into a fully automatic weapon.
In normal semi–auto operation the hammer is cocked by a rearward movement of the bolt carrier. As the carrier moves forward, the hammer is caught and held in the cocked position by the sear located on the forward part of the trigger catching in the sear notch, on the hammer. If you hold the trigger after a shot's fired the sear will not catch in the hammer's sear notch when the hammer cocks because the sear is depressed below the arc of the hammer notch.
This happens because the trigger is being held back by the hook on the disconnector, which is tipped forward and in position to catch the hammer, stopping it from following the bolt carrier forward. When the trigger is released, it allows the hammer to slip from under the disconnector hook and to be caught by the trigger sear in the hammer sear notch, making it necessary to pull the trigger for each shot. As long as the trigger is held back, the sear is held below the arc of the hammer notch. The only thing holding the hammer in the cocked position is the disconnector. The Lightning Link accomplishes full–auto fire by pulling the disconnector to the rear, forcing it to release the hammer.
Complete with full technical diagrams and clear instructions.