He could survive through his strong will and Occlumency skills. By blocking Voldemort from fully penetrating his mind, he can keep his most important secrets safe and hold on to his sanity, which is crucial for survival.
One idea could be that Snape endures the torture while secretly passing on vital information to the Order of the Phoenix through a magical link. Another could be that his past with Voldemort is explored during the torture, like how he first got involved with the Dark Lord and his internal struggle between loyalty to Voldemort in the past and his newfound loyalty to Dumbledore.
In some fanfictions, Snape might be working with Voldemort because he has been blackmailed. Voldemort could have found out some deep, dark secret about Snape and is forcing him to cooperate. This could be related to Snape's past or something he has done that he doesn't want to be made public.
One great fanfiction on this topic is 'The Dark Trials of Snape'. It really delves into Snape's inner turmoil as he endures Voldemort's torture. The author does an excellent job of portraying Snape's complex emotions and his unwavering loyalty to Dumbledore even in the face of such horror.
There's a fanfic where Snape takes advantage of a power struggle within Voldemort's ranks. He poisons Voldemort's drink during a meeting, which causes Voldemort to become severely ill, an injury in a sense. Snape does this to try and disrupt Voldemort's plans and give the Order of the Phoenix more time to prepare their defenses.
Power. Voldemort might see Snape as a means to gain more power. Snape is highly intelligent and skilled in the Dark Arts. By caring for him, Voldemort could be ensuring that Snape remains loyal and continues to use his skills for Voldemort's benefit.
One common theme is loyalty. Snape's loyalty to Dumbledore and the Order of the Phoenix is put to the test during the torture. It shows how he endures for the greater good.
One popular Severus Snape tortured fanfiction is 'The Darkest Days'. It delves into Snape's inner turmoil and the difficult situations he endures in a very engaging way. Another is 'Snape's Struggles' which shows his past traumas and how they shape his character in the fanfic world.
A common plot is that Snape uses his position as a double agent to his advantage. He gets close to Voldemort and then at a key moment, he uses a hidden weapon or spell to injure him. For example, he might have a wand with special properties that he uses against Voldemort. This is popular because it plays on Snape's duplicitous nature.
One possible plotline could be that Snape discovers a hidden weakness of Voldemort. Maybe through his years of spying, he finds out that a certain spell, when combined with an ancient magic artifact, can destroy Voldemort. So he devises a plan to get hold of the artifact and uses the spell at a crucial moment when Voldemort is off - guard, perhaps during a major battle when Voldemort is focused on other opponents.