What does it add?
Context and connective tissue: why something matters, an example, a smooth transition between points. It builds on what you wrote rather than inventing new claims, so the more specific your short version, the better the long one.
When is longer actually better?
Thank-you notes, cover letters, product descriptions, school assignments with a word count, and any reply where a one-liner would read as cold. For texts and quick messages, shorter is usually better, which is what Make it shorter is for.
Expand or Continue writing?
Expand makes what you wrote fuller. Continue writing adds what comes next, the following paragraph or the rest of a story. If you have the full idea but too few words, use Expand. If you have a beginning and need more, use Continue writing.
Where it works
On the TextAI keyboard. Select the short text in Mail, Notes, Slack or a form, tap Expand and the longer version replaces it in place.