"...Hassan had saved us with his slingshot."

Hosseini uses the literary device by stating a flashback. The flashback that Hosseini talks about is when Hassan had saved Amir from Assef by using his slingshot. This is significant because at this moment, Assef is about to rape Hassan and Hassan has no way of defending himself, especially since he does not have his slingshot. Amir could have done something to stop everything, just like Hassan would do for him. But, Amir just watches and does nothing to save Hassan. By using a flashback, it heightens the reader because it allows them to think back on what kind of person Hassan was as oppose to Amir. Though they were "friends" one seemed to have a stronger character than the other. This would explain why Amir feels that he is such a coward, whereas Hassan always stood up for them.