It's automatic. If enough users flag a post, it gets "killed" and will only show up (with the tag "[flagged][dead]") if you have the "showdead" option enabled in your profile.
I don't think it's an awful system but I don't think it's exactly right for HN - in normal operations it works fairly well but when combined with some less desirable HN stereotypes it can just involve shutting down discussion one disagrees with (e.g. merely mentioning Donald Trump by name was an insta flag during the election, but you could always get away with worse if you talked about democrats because the average user is further right than most forums)
I think most of us automatically downvote or flag anything political regardless of personal politics. Especially US presidential politics, because that's what the uninteresting parts of the internet are for.
