From the site of the Spiritualist Church of Canada:
Table tilting is done by spirit employing the use of ectoplasmic rods from a physical medium. The ectoplasm is built up in the body of the physical medium but is actually contributed to by all the sitters and surroundings, as well as a spirit chemist. A rod is attached to the leg of the table and spirit is able to move the table in answer to questions put forth, thereby building up a form of communication. If the table is particularly heavy, the ectoplasm is formed into a lever mechanism to take the weight. At all times this ectoplasm is attached to the medium's body.