How *** Works

[Memory Master]

And what Xigbar said about having to have existed in the place and time you want to travel to not just the time. Here's the link:

Kingdom Hearts 3D: Dream Drop Distance - [Part 20 - The World that Never Was 1/4] (English Subtitles) - YouTube

It's around 4:00.

Yeah, you had to exist at a place and time to travel to that period of time. Regarding the place part of it, you have to have a physical existence there. I don't know what Xigbar is talking about when he mentions the sleeping worlds, as far I know the dream versions of the sleeping worlds are memories of them before they were taken by darkness.

So its a bit confusing but I hope english translations make it clearer when the english version comes out.

 

[Nayru's Love]

Maybe they are taken at the point of when they are destroyed? Like, say for instance, the Ansem we see in 3D was taken at the point right when Ansem was nuked by the light in Kingdom Hearts in KHI. So him being in the future and being absorbed or however into the X-blade wouldn't ruin his past time since he was then destroyed a second later? Same with Xemnas.

As for the rest? perhaps this also works with Xigbar.

XH and just about anyone else taken from their times could have been taken at any point in their lives. Once they return to their original times, they lose their memories of their time-traveling, so it's as if they never even traveled in the first place.