I've been meaning to discuss this issue for a long time, and finally got round to it. "Exformation" is the information you leave out of a message because you know your reader or listener already knows it. But tapping that shared knowledge is the key to powerful fiction. My comments run to about 2,000 words, so you're welcome to Download Exformation in Your Fiction (it's a PDF) and read it at your leisure. I hope you find it useful.