The writing is frequently done by the medium of the Ouija board or crystals or stones. In some cases more than one person must be at the ouiga board. They know historical events and can describe geological and biological items they never were exposed to. Many can leave their work and when they pick it back up, even days later, they do not have to review past work before starting again. They just continue as if there were no break.
Some automatic writers see the words and or pictures of what they are writing in their mind.
This article is well written. It describes several complex books that were written using automatic writing, identifies several well known authors that used it and compared this information to what is known about the way the Book of Mormon was authored. Scott's conclusion is that the Book of Mormon could well have been authored by automatic writing. If you want to do your own reasearch on this subject, after you first read Scott C. Dunn's article, I suggest the following in the order given. Some Mormons have also considered automatic writing.