The mind sifter was a Klingon device used to extract information directly from a prisoner's mind, capable of reducing a subject to a vegetative state.
During the Klingon occupation of Organia in 2267, Commander Kor captured Spock, who was posing as a Vulcan trader, and subjected him to the mind sifter. At lower settings the device could probe memories, while at higher settings it destroyed the mind it examined.
Spock used Vulcan mental discipline to resist it and conceal his true identity, and the occupation ended soon afterward when the Organians revealed their power and prevented war between the Federation and the Klingons.