It's very much of a gimick type of thing. Basically the thin tin fake dust cover goes back and forth in time with each shot. There may be a faint clikety-clack sound...but it's drowned out by the piston hitting the cylinder head.
It does seem durable though. I've put hundreds (600+) of shots through a couple of them and there's zero wear/tear. Nothing seems to be hitting against anything with enough force to damage/wear any part of the blow back piston/spring/assembly/cover.
Kudos to G&G for their design. It is functional (as in it works, but there's no need for it), easy to maintain (there's none needed) and doesn't tear apart the AEG (see the old Guarder auto-back kits).
But the internals are so-so...and the externals leave a lot to be desired (not trying to be too picky...but you're getting what you're paying for certainly).
And the spring release is a nice-touch.