Backstory means to interrupt a narrative in the present in order to go into the past. Whether via flashback, recollection or dialogue, backstory explains why a character is in such a place at a particular moment. Some people don´t know how to do it, others like the late Jacqueline Susann were masters in its usage. However, contemporary consensus says that opening chapters should be grounded strictly in the present.
This rule tends to confuse novice writers who bend over backwards in order to avoid the dreaded pitfall. What they don´t understand is that unless you are beginning with the protagonist’s birth (a la David Copperfield) a little backstory will always creep into those first pages. Even a plot-driven novel will require some characterization, and you have to give characters a little background in order to make them appealing.

By the time I finished Twilight’s first chapter, I didn´t know about the vampires, good and evil, about to cross Bella’s path, but I knew I liked her, and that was due to the backstory that explained her relationship with her parents and why she was on her way to Forks.
Stephanie Meyer found the perfect way to artfully interweave the past into present events. As artfully as Margaret Mitchell introduces her heroine to the reader while subtly letting backstory infiltrate into that first paragraph.
"Scarlett O'Hara was not beautiful, but men seldom realized it when caught by her charm as the Tarleton twins were. In her face were too sharply blended the delicate features of her mother, a Coast aristocrat of French descent, and the heavy ones of her florid Irish father”.

We are instantly interested in Scarlett, but we also care to know about these parents. Coming from such diverse backgrounds, what would bring them together? Does that misalliance affect Scarlett´s nature as well as her physical appearance?
In Jo Graham´s Black Ships, Gull does not start her story with her birth, but with her mother´s circumstances, who she was before and after Troy fell. The same happens in Jacqueline Carey´s Kushiel´s Dart, where Phedre goes to great lengths to tell us all about her pedigree, and her family´s lineage. In both cases, the heroines need the reader to understand their past and how it influences their personalities and lives.
Backstory could serve as a mood enhancer, and so even thrillers and adventure authors use it. Take for example Joseph Kanon´s The Good German that opens with the hero, Jake Geismar, in a flight to post-war Berlin. In that first chapter Jake, recalls his experiences during the war, then goes back further in time, reminiscing the days he spent in that city, covering the 1936 Olympics. That small dosage of backstory helps create a perfect setting and atmosphere for the somber tragedy that is about to unfold.

Of course we know better than to spill the beans in page one. Of course, we are conscious of the importance of withholding juicy tidbits till later, but there are ways in which backstory judiciously employed, could intensify the plot. For example, launching a tale with the past and letting it lead you to the present moment. In Across the Nightingale Floor, the first book in Lian Hearn’s series, Tales of the Otori, Takeo describes, in the first two pages, his relationship with his mother and their life in the mountain village of Mino. On page three, he returns from the forest and finds his village destroyed and his family murdered. The previous introduction adds pathos to this scene and helps us bond with Takeo.
It could be done the other way around. Begin with the present and move into the past. The Amazing Adventures of Kavalier and Clay, Michael Chabon’s Pulitzer winning novel, opens with a common lie that Sam Clay uses to explain the existence of his comic-book character, “The Escapist”. Then the story takes us to 1939 to discover The Escapist’s real genesis. It is a powerful strategic device that lures the reader into the story.

Reading fiction is an emotional experience. I agree that plenty of action keeps your adrenaline pumping and too much exposition makes you doze, but caring about characters from page one, wanting to know more about them, and learning about the circumstances that shaped them is a wonderful way to hook a reader, and backstory could just help you do that.
Is backstory an effective device, even before the 100th page? Do you remember examples where it improved the plot?