Automatic writing has an interesting history. The name contains what it is, just sitting down and writing without trying to influence what comes out with ideas about meaning or story or spelling or even being interesting. It seems to have started with spiritualism, as a means of contacting the dead in the afterlife, in the 1800s. The spiritualist movement was really about women trying to have positions of power in a society that limited them.
So, automatic writing served two purposes. One, the person doing the automatic writing could pretend that the writing was coming from somewhere outside themselves. Two, other people would take it more seriously because they thought the women were just a means for the message to be communicated. By removing the idea of authorship from the writing, both the writer and reader were free to judge the writing for what it was rather than prejudging its source.
The next group to use it were the surrealists who also did automatic drawing. They believe it was a channel to the "genius of creativity" in all of us. Also, they were attracted to the strange images and rhythms it produces.
I don't think automatic writing is a message from another world or the key to your subconscious mind, but it is a useful tool for producing raw material.
Here's a short guide on how to do it.
Find a comfortable place to sit with a flat surface in front of you. Get a pen or pencil and some blank paper. Clear your head, go neutral, then begin writing as fast you can. If you find yourself stopping, just use the last letter of the last sentence you wrote as the first letter of the next sentence and keep going. Don't stop to correct, alter or insert a better idea, this is simply a big dumping ground for you to produce raw material that you can use for whatever you want later.
Write as long as you want, you'll know when it's over.
I recommend putting it aside for a day before reading it. It can be a revelation or a complete disappointment, it doesn't matter because you didn't put any effort into it.
Try it. I promise you'll be entertained. If you feel better pretending that you are channeling a voice from somewhere else, go ahead. That way you don't have to take the blame for what happens. If you decide to do that, you can pretend you are channeling the voices of aliens, fairies, half-dog half-men that live in the swamps of West Virginia, David Lee Roth or President Taft. Maybe it will make it better!
After all, like all good games this has no rules.