Strong screenplays usually pose one central dramatic question that serves as the main story’s through-line otherwise known as the spine of the story.
In the protagonist’s struggle to overcome the obstacles that stand in his/her way of his/her goal, sometimes there is a fumbled effort, or a moment where new hard-won knowledge is obtained that causes the original central dramatic question of the piece to be dramatically changed.
That change is the plot twist or reversal.
WIZARD OF OZ
Will Dorothy find her way back to Kansas? It’s the original central dramatic question that acted as the spine to the story.
Once Dorothy finally gets into the sacred, secret hall to meet The Wiz, he throws Dorothy a curve.
She wants him to help her get back home. To get that help, The Wiz tells Dorothy she must first do away with the Wicked Witch of the West.
Will Dorothy be successful in dispatching the Wicked Witch? Or will she die in the trying?
It’s a new central dramatic question that now acts as the spine to the story.
That’s a plot twist.