Originally posted by Joel
Well get this, she's the only one in the story who can use the power to posess somebodys spirit that she knows and travel back in time to see if she can change the past, Im guessing her powers seemed to grow over time so she would just need to look into the past and instantly posess somebody. This is just my theory.
Like Shiva said, there was Junction Machine Ellone. IT does say that that is what Ultemecia used to send her consiousness back and possess sorceresses of the past (Edea, Adel, Rinoa)

There is also another good piece of evidence to look out for which supports the rinultimecia theory (my name for Rinoa-Ultimecia ^_^)
When you fight Griever at the end.... look closely at his explosion once you beat him, before Ultemecia junctions herself to him. You'll notice that when he explodes, white feathers float about the place. Hard to see, yes, but look, they are there.
And I believe we all know which character in the game is associated with white feathers.