The Preservers obelisk was a statue-like structure left by the Preservers on Amerind. It consisted of two parts, a base just under two meters in height, and a larger metallic fixture approximately 7.4 meters in height. Constructed of a material opaque to sensors, it resisted analysis. Its surface was partially covered by glyphs.
In 2268, Captain Kirk of the USS Enterprise inadvertently hit on the tonal combination that opened the access hatch atop the base. As he was standing on the hatch at the time, this pitched him down the stairs into the base. Stunned, he attempted to regain his footing but succeeded only in incorrectly triggering a memory beam designed to instruct a user in the operation of the machinery contained within the obelisk - a powerful asteroid deflector. This robbed him of his memory. Because no other member of the landing party saw this accident or could discover their captain's whereabouts, they were forced to leave him on Amerind for several months while the Enterprise attempted to divert an asteroid on a collision course with Amerind.
Spock eventually discovered how to interpret the alien language, and learned the purpose of the structure. Returning to Amerind, he was able to restore Kirk's memories and between them, they discovered how to operate the deflector and save the people of Amerind. During his stay there, Kirk learned that the Preservers, whom the natives called "Wise Ones", left the secret of operating the deflector with one member of the tribe, the medicine man. In 2268, a man named Salish held that office. Salish's father did not wish to share his knowledge too soon, ultimately dying before he did, so Salish could not save his people.
The deflector mechanism was considerably more powerful than the deflection beams the Enterprise could generate with full engine power. It easily pushed the asteroid away with just a few seconds of contact. (TOS: "The Paradise Syndrome")