In the middle of Quentin’s segment of narration, he remembers a conversation he had with Caddy in which she obliquely discussed her affairs with men. She told him:
When they touched me I died
This recollection, which triggers a larger memory of Quentin’s fight with Dalton Ames, is a key moment of intensity to Quentin. Here, his sister is unusually vulnerable, which causes a powerful reaction from Quentin. The line is honest, despairing, melodramatic, and open. It’s also a hyperbole: Caddy doesn’t actually die when she has relations with men.
This line is a great example of why literary devices are important. Caddy is being hyperbolic, but her hyperbole is far more effective at expressing the truth of her emotions than factual or literal language would be. By speaking figuratively, Caddy is far more able to convey the intensity of her emotional experience; and readers are thus more capable of understanding both her emotionally fraught lifestyle and Quentin’s charged response to her habits. Hyperbole conveys the stakes of the moment in a way literal language could never achieve.