choH'a' was a form of species reassignment protocol used by the Klingon House of Mo'Kai during the Federation-Klingon War. It was specifically designed to gather intelligence on Starfleet through infiltration. (DIS: "The War Without, The War Within") In short, it involved surgically modifying a Klingon to look like a donor Human, whose DNA, memories and consciousnesses were also harvested and added on top of the Klingon identity, which was nonetheless expected to remain intact and waiting to be reactivated.
The method was not detectable by Starfleet even with a full set of physical scans, which did find masses of scar tissue surrounding the organs, but attributed them to torture. Nor was the Klingon personality detected by the highly effective manchurian test, which aimed to detect what used to be called brainwashing: commands or personality engrams underneath a functioning consciousness. However, more advanced testing, including Chondroblast-cell scans, did reveal a more troubling picture, hinting at procedures like marrow-diminutions or neurological therapy. (DIS: "Despite Yourself")
The only confirmed application of the protocol was the transformation of Voq in 2256, for the purposes of infiltrating USS Discovery and learning its secrets. When asked if there were others, a House of Mo'Kai operative involved in the operation claimed that he was the test case. (DIS: "The Wolf Inside", "The War Without, The War Within")
The Klingon subject for the procedure, Voq, was then modified into a "shell" that resembled Tyler. This involved traumatic surgery, including the flaying of his skin and sawing his heart into pieces. His limbs were opened up and his Radius, femurs and even his Spinal cord were shortened. The tips of his fingers were shaved down, his bones cracked open and crushed. As a result of these procedures, scar tissue surrounded all organs. (DIS: "Despite Yourself", "Vaulting Ambition", "The War Without, The War Within")
Finaly, Voq's psyche was then grafted into Tyler's. Doctor Hugh Culber theorized that a new personality might have been placed on top of the original one, with the original personality still intact underneath. (DIS: "Despite Yourself", "Vaulting Ambition")
Following the procedure, the new Tyler was successfully inserted on the Discovery as planned. (DIS: "Choose Your Pain", "Lethe") However, even while still believing he was Tyler, memories of the surgery resurfaced. Tyler mistook them for torture and started to exhibit symptoms that were recognized as PTSD. (DIS: "Into the Forest I Go") Later L'Rell attempted to re-active his Klingon personality through a Klingon prayer. This went awry, leaving the Human and Klingon personalities to war and causing mental anguish. Tyler nearly died, until L'Rell purged Voq's mind from Tyler's brain. (DIS: "Despite Yourself", "Vaulting Ambition")