Quote Originally Posted by Serapy View Post
I'm puzzled as to how Time Decompression used this power source to initialize the necessary procedures. Or it's just that Ultimecia's castle ended up in the process of becoming destroyed and hence the Green light somehow appearing right in front of Squall's eyes.
It's neither.

It's symbolic for the player to view the entirety of what Squall and his friends achieved.

Also, the Time Decompression drew magic from Ultimecia right before she was offed in order to power it.