When it is, the file is committed atomically and wholly in one piece. Behaving in a way very like NTFS file streams, Transactional NTFS commits file changes to some alternate cache until the change is guaranteed.
The truth is file streams are weird, and they confuse me, but apparently it gets deeper. So, I really didn't think I would be researching NTFS this morning, but, thanks mostly to comments below, I learned something.
