@ notsurewheretogo,
"These materializations occur by a power which the demons posess of being able to draw out of the mediums body connected streams of the cells of which her body is composed, and then changing the form of these living cells into any desired appearance."
In the world of the paranormal that is known as ectoplasm. The image below shows what these 'spirit mediums' are doing. It's fake of course and the Witchtower falls for it.