It's the belt feed mechanism. See here;
They're probably using the sleeze because the gun was attached to it securely and it gives a good place to mount it on their improvised tripod. Not that I'd want to be anywhere around when the tried to fire it. Especially behind it.
But it brings up a good point; if it was originally an electric motor feed mechanism, how they feeding it here? Could they have replaced the electric motor with a hand crank? Beyond my realm of knowledge...but would make the pic even more interesting if so.