If You Can't Demo It, You Haven't Defined It
There is a familiar kind of spec that reads beautifully and means nothing. It is full of words like "seamless," "intuitive," and "the user can easily manage their settings." Everyone nods in the review. Everyone leaves with a slightly different picture in their head. Three weeks later you discover that the thing engineering built and the thing you imagined share a name and nothing else. The document was clear. The product was not. Those are not the same kind of clarity.
The most reliable test of whether you have actually defined something is brutally simple: can you describe the demo? Not the architecture, not the requirements. The demo. What does the person click, what do they see, what happens next, and how do they know it worked? If you cannot narrate that as a concrete sequence of moments, you have not defined the feature. You have described a wish.
This is why working software beats perfect documents. A document can stay vague indefinitely because prose tolerates ambiguity. A running screen cannot. The software has to do something specific when you click the button. There is no "seamless" in a real interface; there is only the actual transition that actually happens. The act of making it demoable is the act of resolving every fuzzy decision you were quietly deferring.
So flip the order. Start from the demo and work backward into the spec, rather than writing the spec and hoping a demo falls out. Walk through the literal sequence a user experiences. The instant you do, the holes appear: what happens when this is empty, what does the error say, where does the user land when they are done. These are exactly the questions a prose spec lets you skip and a demo forces you to answer.
This discipline also fixes your reviews. "Show me the demo" is a far better checkpoint than "is the spec approved," because a demo cannot hide behind adjectives. Either the thing works in front of you or it does not. Either it is obvious what it does or it raises questions. Demos collapse the distance between what you said and what is true, and that distance is where most product failures live.
The spec is where ambiguity hides. The demo is where it dies.