A mind scanner was a 23rd century Klingon interrogation machine that functioned as both a tool and a weapon used for the sole purpose of being a truth finder. Though the mind scanner was thought to be "somewhat drastic, but very efficient," it was considered a more suitable alternative to "such crude methods" as drugs.
The device was operated by reaching directly into a subject's mind and recording every thought and bit of knowledge within. Designed with variable settings, which determined the amount of force used, it could be a mind sifter or a mind ripper to its subject. If too much force was used, it would cause brain damage leaving the mind permanently emptied and the subject a "mental vegetable."
In 2267, while trapped on the Organia at the outbreak of the Federation-Klingon War, Commander Kor, the military governor of the planet, set up a mind scanner in an examination room at the citadel that served as his headquarters.
When Kor discovered James T. Kirk (as Baroner) and Spock (as a Vulcan merchant) among his subjects, he ordered his lieutenant to have the Vulcan examined, warning that if it was revealed that Spock was lying about his true nature, he would die. Following his examination, it was revealed that Spock's mind was "remarkably disciplined", even under the Force Four setting, which was sufficiently capable of breaking down any pretense.
Though unknown to the Klingons at the time, Vulcan mental disciplines were found to be more resistant to use of the mind scanner, and allowed them to maintain a shield. Spock described the use of the mind scanner on him as "an interesting experience," and while Kirk dismissed that it "can't be all the terror they think it is," Spock told him that "it should not be underestimated," stating that "those without a highly disciplined mind were not protected."
When it was later discovered that Spock and Baroner were responsible for the sabotage performed outside Kor's headquarters, Kor threatened to use the device on Baroner, stating that "he will be killed after he has had first-hand experience of our mind scanner," until the Organian Councilor Ayelborne explained that there was "no need to use your machine" and that the two were in fact agents of Starfleet.
Kor first attempted to coerce strategic information from Kirk the old-fashioned way, by having a "talk," but when Kirk refused, Kor threatened to explore the reason behind his ability to block the scanner through dissection. However, before Kor could carry out his threat, the conflict between the Klingons and the Federation was halted through intervention from the Organians. (TOS: "Errand of Mercy")