Hijacking is a type of memory alteration and fear conditioning method developed by the Capitol for torture. Tracker jacker venom is used to associate fear with certain memories, and to alter those memories with hallucinations. It is difficult, but not impossible, to fully cure. It was most notably used on Peeta Mellark during the rebellion in Mockingjay.
Process and initial effects
A certain memory is first called to the front of the mind by some type of stimulus. Words and images associated with the desired memory can be used; in Peeta Mellark's case, recordings of events in the arena were utilized when possible. Once the victim has remembered the event, tracker jacker venom is injected into the bloodstream. The quantity of venom is small enough that it causes almost no physical reaction (pain, swelling, etc.) and minimal hallucination. However, the memory is slightly altered by the hallucinatory effects and becomes subconsciously associated with fear and pain. The disorienting effects help to camouflage the falsity of the memories. Afterwards, the brain stores the memory in the new form, along with the negative associations.
Complete hijacking requires many sessions. Peeta, after months of treatment, was still able to resist the effects. He warned Katniss Everdeen of the Capitol's plans to attack her, even though his memories about her had been hijacked. Only a short time later, his hijacking had been completed.
A while after Peeta regained consciousness, Gale, Boggs, and a few other District 13 soldiers went to the Capitol, found him, and helped him to recover. Katniss was told about Peeta's arrival. She flew all the way to the medical ward, where she saw Gale in a room and ran to get to him, but they closed the door. So she carried on until she saw Peeta's room. He had already woken and she was sad because she wanted to be alone with him and for her to be the first for Peeta to see. When Peeta saw her, he brushed away the medics. Katniss ran to him as did Peeta with his arms out in front of him. Then Peeta stopped and Katniss was just about to embrace him when his hands locked around her neck and he tried to strangle her.
Long-term effects and recovery
All the below information is based on Peeta Mellark, as he is the only known person to have survived the procedure. It is possible, even likely, that other cases would have dramatically different results.
Although it is not specified in the text what was the most successful treatment, Primrose Everdeen suggested "reverse hijacking," in which Peeta received a dose of calming morphling and was exposed to happy memories of Katniss. The first time the doctors tried it, however, Peeta appeared dazed and stunned for several hours.
Immediately after his rescue, Peeta assumed the altered memories were true and behaved accordingly. He was usually unreasonable and aggressive, and could not calm himself enough to make clear decisions. When he was reunited with Katniss, his brain registred her as a severe threat because of altered memories, and he tried to kill her on sight.
Peeta must have communicated somewhat, or at least understood what was said to him, because he mentioned having been told that he was in District 13. After time and work from psychologists and doctors, he was able to have a normal conversation for a few minutes with Delly Cartwright. She was chosen because she was comfortingly familiar and liked by Peeta, but was not a part of any hijacked memories. However, he quickly brought Katniss into the conversation and within seconds was screaming and raving again.
After a while he had enough self-control and rationality to talk to Katniss, but was bitter, critical, and inconsiderate. His questions showed that he was no longer sure of anything before the time of his rescue. Later he was allowed to eat with the rest of District 13, but was handcuffed and accompanied by guards. His personality still had not returned to normal at this point.
Later, Peeta joined Squad 451, where he eventually began to play "real or not real" and showed improvement. However, when he was faced with a stressful situation, he completely lost control, trying to attack Katniss and killed Mitchell by kicking him into a pod. Once he was stable again, Peeta seemed depressed and asked to be killed. For a long time afterwards he did not trust himself and refused to let the others remove his handcuffs.
Eventually his personality went back to normal, and he never lost his grip on reality again. But even years afterwards, he still experienced flashbacks, and had to grab onto something solid while he waited for them to pass.
Distinguishing altered memories
Because of the hallucinatory effects of the tracker jacker venom, the memories become "shiny," in Peeta's words. After Peeta states this, Katniss remembers that when stung by the tracker jackers in 74th Hunger Games, she thought she fell into a pit of shiny orange bubbles. This sometimes allows the victim to tell when the memory has been tampered with, but is not necessarily reliable.
Jackson created a game to sort through Peeta's memories, which he played with the other members of Squad 451. He would mention a memory or fact, and ask if it was "real or not real." If not answered, he could quickly become frustrated.
Hijacking means to seize a vehicle or to steal from it after it has been seized. It usually refers to the unlawful capture of an aircraft.
The term, as it is used in the book, is a play on "tracker jacker," which itself comes from "yellow jacket."