The master switch for the Tantalus Colony's hyper-power circuits
Hyper-power circuits were an energy-generation technology utilizing megavoltage, used to power such consumptive devices as the security screens used for penal colonies, which were in almost continuously operation. The Tantalus Penal Colony was one such facility that utilized hyper-power circuits.
In 2266, Doctor Helen Noel was successful in powering down the circuits in order to deactivate the security screen surrounding the colony, allowing Mister Spock and a security team to beam down and take Doctor Tristan Adams into custody, as he had become dangerously unstable. (TOS: "Dagger of the Mind")