Automatic writing or Automatic drawing is a form of channelling, and is very similar to the trance that a medium uses for a seance.
I have only dabbled with it.
When I was a child we used to do a creative game called one line. My Mother or grandmother would draw a line, then I would go into the meditative state and complete the drawing. It was only a game to me, and at the time I did not realize they were training me for automatic writing.
I have had very minute success with the writing.
I have drawn, and painted in trance, and it is interesting to try to create what you were compelled to paint while in that state. All the of the stuff I have painted have had the same theme. They end up images of one of the levels of The I Ching. I have never been able to understand why, but I am personally comforted by these images.
I usually do not channel spirits so no spirit takes over my body to tell a story. The only one I have ever channeled is myself in a past life. I actually talk to spirits, or can compel answers from spirits through my own spirit guides.
Spirits don't find me a good conduit to actually use my body for their purposes.
I know a woman who is an automatic drawer. Her work is cryptic. She actually has compiled a "dictionary" of her images and she is well studied in the past art of language as pictures from history. Like hieroglyphs.
Automatic writing is very amazing. Those that do it create their works while looking out into the space of their inner mind while a spirit channels through them. This means that spirits are able to use the medium's body to do their bidding for a short time.
This is not for every medium. It means giving up control to the spirit, and having the spiritual strength to regain control again. This requires a lot of personal work on the medium's part. Years of deep meditation, strong faith that never wavers, and good spirit guides.
When you read of folks who channel volumes of work, one must think of the dedication that medium must give to her faith to be able to accomplish such a thing.
Most mediums try to do automatic writing at some point. Curiousity to see if they can do it is what starts it. If the medium has an affinity for it, they will know it in time, and they will work to master that state of mind necessary to channel the writing.
If you compare automatic writing to Ouija, or Pendulum dowsing they are things that should be left to only the most knowledgeable. Not to be done half-heartedly, as to channel means to allow another spirit within your body to use it as a vessel, and can be dangerous if not done correctly.