User blog comment:Seieireppa/God Eater:Universitas, chapter 47: Wings Towards Tomorrow (Arc 2 final chapter)/@comment-1419424-20150729023551/@comment-1419424-20150729025339

I'm going to use an example from Virtue's Last Reward.

"Suppose 100 people are biking down a road, when all of a sudden they come to a fork.  They all agree to go left, but somewhere along the way, 90 of those 100 people die.  You are among the survivors, and you have the power to go back in time and warn them.  You do this, and the group takes the right fork instead, surviving with all the members intact.  However, the other future, the one where 90 people died, still occurred.  You merely took a different path."

It's the creation of a new future, essentially, not the alteration of the existing future.

(God, I probably butchered that quote...)