The pivot in any plotline is often The Reveal. A character is revealed as another character's mother, a god, or secret suitor or arch nemesis in disguise. More broadly, the audience is given new information which had been withheld to create suspense. The Reveal changes the nature of the plot, often pushing it from suspense towards action. A good reveal will also create a new set of questions and further suspense.
Luke, I Am Your Father
Darth Vader: If you only knew the power of The Dark Side. Obi-Wan never told you what happened to your father.
Luke Skywalker: He told me enough! He told me you killed him!
Darth Vader: No. I am your father.
Luke Skywalker: No...that's not true! That's impossible!
Darth Vader: Search your feelings. You know it to be true.
Luke Skywalker: NOOOOOOOOO! NOOOOOOOO!
- Star Wars Episode V: The Empire Strikes Back