"Do shut up."

I have a question about the British (I believe) habit of putting "do" in front of imperatives. I'm not sure why, but as an American it makes it seem like the imperative is meant to be more polite - it might just be association with Britain, which automatically makes everything seem more polite, or it might be something else. Anyway, it doesn't seem to work that way in British English. I found the quote in the title in a book written by a British author and it had me in hysterics because it's obviously meant to be rude, but then there is this polite little "do" at the beginning of it, and I don't think it was meant to be sarcastic.

So, my question is - what is the "do" doing there if it's not making the imperative polite? Is it just a way to make the imperative more forceful, or something?