[Darklen]

It's an electric pneumatic blowback. There's a secondary piston assembly on top of the main piston. As the main piston fires, air is tapped off though a small hole in the top of the cylinder that goes to the blowback cylinder. the only thing the blowback cylinder does is drive the faux bolt back and forth. Doesn't look like much when you dry fire with the gun empty but look decent with it loaded and firing. Depending on the gun it makes a snappy cracking sound or more of a rattling snap (like the AK)

If you don't like it, it's fairly easy to disable but requires disassembly of the mechbox to do so.