Sorry, my bad. I automatically make assumptions I shouldn't.

I comes from ancient Greek plays where, at the end, a god or the like comes down and saves the hero. It means "mechanics from a god", and basically is a get out of gaol free card for a hero.

Think the ending of Jurassic Park, the original film/book, where the heroes are under attack from the velociraptors and suddenly the T.rex appears and attacks only the other dinosaurs, and not the people, so the people can flee and survive. Yay! But deus ex machina.