Pontimer's theory of what the Fist of Guthix is that the god Guthix found something that was dangerous within the caves and used his power to protect it. He believes that the imprint that remains within the caves is a magical ward that Guthix left to guard the site. His theory is probably correct, as the imprint on the ground matches the design of the Stone of Jas, and the Stone of Jas is extremely powerful, a slight touch drastically increases the stats of a player. Alran and Getorix also have theories about this.