What it is: Making a rhetorical argument against an argument you have made or otherwise believe in, in order to further discuss the weaknesses of your argument so that you may strengthen it.
What it is not: Going into a space where your personal viewpoint is specifically the opposite of the viewpoint of regular members and proceeding to argue, without regard to etiquette or basic rules of rhetoric, that they are wrong.
If you are going to do the latter, then do not say you are playing devil’s advocate. You are, at best, going to annoy the shit out of someone. (Of course, in my experience, those playing “devil’s advocate” in such a way are overwhelmingly trying to use the phrase as a smokescreen for the fact that they are trolling with intent to cause maximum harm to any given community.)