Saw III

The aftermath of a Jigsaw game is discovered by a SWAT team. The victim, Troy, was meant to rip chains from his body in order to escape from a bomb. Lieutenant Daniel Rigg calls Detectives Mark Hoffman and Allison Kerry to the scene. Kerry, who is guilt-ridden over the disappearance of Eric Matthews, points out that the room's only exit was welded shut, breaking Jigsaw's modus operandi of giving his victims a chance to survive. That night, while reviewing the videotape, she is abducted and awakens in a harness that is hooked into her ribs. She manages to retrieve the key from a suspended beaker of acid and unlock it before the timer expires, but it tears her rib cage apart anyway, killing her.
Now bedridden from cancer, John Kramer instructs Amanda Young to abduct Dr. Lynn Denlon from her hospital. She is brought to Kramer and instructed to keep him alive until the other victim has completed his game. As Kramer explains the rules, Amanda locks a collar around Lynn's throat that connects to Kramer's heart rate monitor and will detonate if Kramer dies or Lynn moves out of range. The other victim, Jeff, awakens in a box in an abandoned meatpacking plant and learns from a microcassette recorder that he must undergo tests which will lead him to "the man responsible for the loss of his child". Jeff has become unstable and vengeful in the three years since his son Dylan was killed by a drunk driver, and is now estranged from his wife and neglects his daughter Corbett.
Jeff's first test leads him into a meat freezer. He finds Danica Scott, the only witness of Dylan's death, who refused to testify in court; she is naked and chained at the wrists between two poles which begin spraying ice-cold water at random intervals. She manages to convince Jeff to help her, but freezes to death before he can retrieve the key. His second test leads him to a vat, at the bottom of which Judge Halden, who gave Dylan's killer six months in jail, is chained at the neck. The vat slowly fills with liquefied pig corpses, but Jeff reluctantly incinerates Dylan's belongings in order to retrieve the key and save Halden. His third test brings him before Timothy Young, Dylan's killer, who is strapped to a machine that will twist his limbs and neck in turn until they break. The key is tied to the trigger of an enclosed shotgun. Jeff eventually tries to free Timothy, but taking the key discharges the shotgun and accidentally kills Halden, and Jeff is unable to stop the machine before Timothy's neck breaks.
During an improvised surgery, in which Lynn removes a piece of Kramer's skull to relieve pressure on his brain, he hallucinates about another woman and declares his love out loud, distressing Amanda. She leaves, and flashbacks show her time with Kramer, as well as her abduction of Adam, which left her guilt-ridden to the point she gave him a mercy killing out of guilt some time after his game. In the present, she reads a letter addressed to her which drives her to hysterics. As Lynn confesses to Kramer that her ordeal has given her new appreciation of her family, Amanda returns with the news that Jeff has completed his tests, but refuses to remove Lynn's collar. She reveals that she no longer believes in Kramer's philosophy, and has designed her traps simply to kill, including Troy's and Kerry's. She also fought with Eric Matthews after he escaped the bathroom by breaking his foot; she implies killing him, but Kramer believes she left him to die.
Refusing to listen to Kramer's warnings, Amanda shoots Lynn in the back just as Jeff arrives. He retaliates by shooting Amanda in the neck with a gun provided by Kramer during his tests. As Amanda slowly dies, Kramer reveals that Lynn's test was actually hers: aware of her modus operandi, and unwilling to allow a murderer to inherit his legacy, he decided to test "her will to keep someone alive"; to this end, he told her nothing about Jeff and Lynn, including their marriage. Kramer then addresses Jeff, offering to call an ambulance for Lynn if he accepts one final test: he can choose to kill Kramer or forgive him. Jeff tells Kramer he forgives him, but then slashes his throat with a power saw. The door to the sickroom seals itself as a dying Kramer plays a final tape, which tells Jeff that he has failed his test by killing Kramer, the only person who knows Corbett's whereabouts; to save her, Jeff must play another game. The tape ends as Kramer dies, and the collar promptly detonates and kills Lynn, leaving a screaming Jeff trapped in the sickroom with the three corpses.

Question: Reading what drives Lynn hysterics?
Answer: letter