Board Thread:General Discussion/@comment-24589671-20140329051413/@comment-6175354-20140407175954

@Arcutucrus:

Like when Harry and Hermione went back in time, to save Sirius Black. Harry was in two places at once, when he cast the Patronus.... he saved his past self.

If the time-travel theory is true for the season finale, then Emma and Hook would be the ones "fixing" the timeline. They are the only main characters who do not run the risk, of meeting their past selves. As Emma was not yet born, and Hook was still in Neverland. By going back, they would be ensuring that the past events happen correctly.