The Ending... spoilers

I assume this is explained on here, but I don't have time to look right now and am being sure I don't forget what iw ant to know later on. lol. If it's in another thread I assume I will find it when I have time.

Apparently the frantic ending was too fast for me to realize what happened on the 1969 end. I understand what happened to allow the dad to know he needed to save his son in the future, but what in the world happened to change the mother's death to not happening? I noticed nothing at all that future John did to change anything. He just says the mother still dies then boom he has now ay to help anymore and somehow magically she doesn't die after all.

Of course the way time travel worked in this movie is it seemed like something HAD TO HAPPEN in the past before it changed the future. So, in other words, even if the mother was NOT going to die, the actual event that prevents it had to happen before it was felt in the future... that's all I can figure, unless I missed something.

I have a feeling if I think too much I am going to find a lot of flaws in this movie's idea of time travel, but it was still a fun movie and I think technically you could have excuses to get out of potential plot holes.