How did James Potter save Harry and Sirius Black from the dementors?

Answer 1

Technically, James himself didn't save them. READERS BEWARE: SPOILERS CONTAINED FROM THE END OF Harry Potter AND THE PRISONER OF AZKABAN! Through the use of Hermione's Time Turner, Harry traveled back three hours to save Sirius from being executed by the Ministry of Magic by the dementor's kiss. After saving Buckbeak from execution by decapitation, and after escaping from the werewolf form of Remus Lupin, Harry and Hermione headed to Hagrid's deserted hut (Hagrid having gone off for a drink). Harry, who had witnessed being saved by the dementors by a stranger with a four-legged Patronus whom he believed to be his father, James, went off to see him appear and save his past self and Sirius. When it was almost too late, Harry realized that he hadn't seen his father, but himself (since Harry was the spitting image of his father, save his eyes, which came from his mother, Lily). He conjured the Patronus and chased off the dementors, and when the creature returned to him, he saw that it was a stag. He had been informed a few hours earlier that to maintain their friendship to Remus, his three closest friends Peter Pettigrew, Sirius Black, and James Potter, became Animagi, each with a nickname corresponding to their animal, with Remus as Moony, Peter as Wormtail (rat), Sirius as Padfoot (large black dog), and James as Prongs. Harry realized at that moment that his Patronus took the form of a stag since it was his father's animal. END SPOILERS! So, to summarize, it was actually Harry who saved Harry and Sirius from the dementors, with James only appearing via Harry's Patronus.

Answer 2

she asked Voldemort to kill her instead of Harry. She wouldn't move out of the way even though it killed her. That sacrifice protected Harry because her love for him was too pure for dark magic.