Before I write a spec, I write the release note. Three sentences, in the words a customer would use: what changed, who it's for, and what they can stop doing.
It sounds like a writing trick. It's a scoping tool. When the note needs a fourth sentence, the project is usually two projects. When I can't name what the customer can stop doing, we haven't found the problem yet.
How I use it
The note goes at the top of the spec, before any requirements.
Engineers and design review the note first. Most scope arguments happen here, where they're cheap.
On launch day, the note we ship is usually 80% of the first draft.
It won't fix a bad idea. It will tell you sooner that you have one.