User blog comment:Zeldakasumi/World Line Theory, Divergence, Loops, and Kurisu's Deaths and Salvation/@comment-27281875-20160322212644

So that means the Turning Point, where the worldline changed for the first time, is when Okabe in 7/28 sent his very first D-Mail about Kurisu's Death. And the cause of that is because Okabe in 7/28 observed Kurisu lying unconscious on a pool of blood.

And because of what his past version have observed, he cannot undo it, and then there will be convergence.

But the key to change the past is not to undo what he has observed, instead is to change things which is hidden from his observation. As long as the outcome is the same observation his past version had, then changing the past is possible.

Suzuha and Okabe returns to 8/21, but they departed from 7/28 before the Turning Point happened, so the moment they depart the worldline still not changed, that explains why Suzuha and the time machine didn't disappear just right after Okabe from 8/21 finished his work on deceiving his past self and deceiving the world. But as soon as they arrive in 8/21, the worldline already changed to the worldline which holds a different present and different future.

hmm, this is getting nowhere