Knuth: The first rule of writing is to understand your audience—the better you know your reader the better you can write, of course. The second rule, for technical writing, is say everything twice in complementary ways so that the person who's reading it has a chance to put the ideas into his or her brain in ways that reinforce each other. So in technical writing usually there's redundancy. Things are said both formally and informally. Or you give a definition and then you say, “Therefore, such and such is true,” which you can only understand if you've understood the definition.
