Before anything is scheduled we ask for one page. Not a specification — a page. What is happening now, what you wish were happening instead, and who would notice if it changed.
The constraint is the useful part. A page forces a decision about what the problem actually is, and the decision is far more valuable to us than the detail would have been. Long briefs describe systems. Short briefs describe intent.
If the page is hard to write, that is information too, and it usually means the process has more than one owner. We would rather find that out before anyone has paid for a workshop.