A katric transfer was the switching of Vulcan katras between two individuals, effectively a "body swap." Doctor Joseph M'Benga considered them exceedingly rare, and noted by bridging the gap between medicine and the mind, emotion plays a part.
In 2259, Spock suggested he and T'Pring partake in a Vulcan soul-sharing to better understand each other's perspective and improve their relationship. At the conclusion of the ritual, something went wrong, and the two found that they were inhabiting each other's bodies. After hours of meditation and adjustments, they found they had to live a day in each other's lives.
Dr. M'Benga, experienced in Vulcan medicine, theorized and successfully reversed the effects by applying ground Nivallan sea urchin paste to the foreheads of both Spock and T'Pring, for its match to the natural electromagnetism of Vulcan skin. He then applied cortical stimulation through a series of crystals placed in the paste, matching the original brain patterns of the host bodies. Both Spock and T'Pring found the experience painful, but effective. (SNW: "Spock Amok")