One of the more WTF, don't-try-this-at-home weapon hybrids we've come across in a while.
Here's a quick explaination via Everyday No Days Off:
Because of the crappy portrait style video from far away I can’t really 100% tell what’s going on. It looks to me like there’s some type of plunger at the back of the shotgun, which is likely in contact with the firing pin. The cable then whacks that and… BOOM.
This guy and his crazy bow-gun should be a guest on the best FPS Russia video.