What is spirit writing?

The Process

Slates are thoroughly washed and dried. A small slate pencil is placed between two slates, a string is tied around them, and they are laid on the table in full view in bright light. While the writing is being produced, the medium requests the observer to hold the slates in his hands while he himself holds the other side. 

One hears spirit hands, writing in their own handwriting, characteristic messages with incredible rapidity.