"Stuart and Schele have noted that the base of the stairway itself represents an elaborate inverted Tlaloc head. The figures and entire stairway inscriptions are being belched forth out of the open mouth of this beast, whose lower jaw was at the top of the steps. According to Stuart the figures were being made to appear in a vision produced by the auto-sacrifice of the living ruler. [A] cache was placed inside the head of the inverted Tlaloc, precisely where its brain would be located. The auto-sacrificial rite was thus the engine behind the production of the vision."
William L. Fash, Scribes, Warriors and Kings: The City of Copán and the Ancient Maya, p. 148-49