Commonly known as the “Lightning Link” the S.W.D., Inc. Auto Connector is a rarity to see these days. With fewer than 900 manufactured and registered in the NFA registry, the Lightning Links are a novel way to convert a semi-automatic AR-15 into a fully-automatic firearm. While the conversion is two-way (meaning can be installed and removed without issue), the Lightning Link will turn the firearm into full-auto only as installed.
Its physical operation is interesting for those looking at weapons design. Rather than require modified hammers and other full-auto parts, the Lightning Link was designed for off-the-shelf commercial AR-15 rifles without the sear block. It normally consist of two parts, the main link and the sear.
The kit requires the use of a true AR-15 semi-auto bolt carrier group. Installed, it hooks around the disconnector and the rear portion fits behind the lug of the upper receiver using the lug as a lever point. As the bolt carrier returns to battery, the rear portion of the bolt carrier group catches the vertical portion of the link, causing it to pitch forward pulling the link to the rear which in turns pulls the disconnector to the rear releasing the hammer for the follow-on ignition.
YouTube user “Magoo6541” has a great (if older) video showing a real link installed and a few videos with one working in a firearm for those wanting a video explanation: