Readers connect to the story through the character.
Readers identify with the characters through the characters' emotions.
During the consultation, the back-story develops in such a way to bring more emotionality into the story as the protagonist reacts, sometimes inappropriately, based on the pain she suffered as child and continues to feel, though basically unconsciously so.
Not wanting to give the writer's story away, I give examples here to show you what I hope to convey.
To begin with, the back-story is the time when and the reason why the protagonist's innocence is lost. This can be at a young age years before the actual story begins or more recently but still before the actual front story begins.
This defining moment may be something the character overheard and took to heart and has lived her life by ever since. This moment may be some sort of negative treatment toward the protagonist earlier in her life that she now carries with her, unconsciously or not, and that interferes with her capability to achieve that which she most longs for.
Back-story provides brilliant motivation for the character's actions even when the character is virtually unaware of it or believes her motivation comes from elsewhere.
In other words, if the protagonist was betrayed at some point in her past (back-story), she from that point forward feels betrayal in the actions of others now in the front story. If she was abandoned in her back-story, she feels the same pain she felt as a child even now as an adult and even if the current behavior by another is actually benign and insignificant to others but never to her. Perhaps she was lied to, physically abused, emotionally bullied, neglected... you fill in the blank for your character.
This WOUND then becomes what she must become CONSCIOUS of and overcome in order for the sense of perfection to be restored in the final 1/4 of the story.
The back-story never has to be revealed in the story at all. However, so long as you as the writer knows the protagonist's back-story, you then have compelling motivation for the protagonist's actions in the front story.
Just remember, the back-story is not the story.
The front story made up of the primary plot and that is the story.
Plot Series: How Do I Plot a Novel, Memoir, Screenplay? is playing on my YouTube channel. Currently, there are 22 steps. A directory of the program is to your right. Each link takes you to a video that explains that particular plot concept about the universal story.