A scene in a movie, novel, etc., set in a time earlier than the main story. A sudden and disturbing vivid memory of an event in the past, typically as the result of psychological trauma or taking LSD. In a series of flashbacks we follow the pair through their teenage years.
My two stories of Flashback are, Maze runner, and Shrek. In Shrek the flashback happened when the Captain Hooke pointed at Gingerbread Man and told him to remember how he was made. In Maze runner Thomas was sleeping and he woke up and had a flashback of him working as the creators.
My own resource of my choosing or creation is movies.