As to how the Eluvians work, this is what I understood. Imshael created a gem that could reawaken all dormant eluvians, but only if you placed the gem in a pedestal at the heart of the network, which seems to be in an unknown location. The group of Celene, Michel, Briala, and Felassan are able to use the red gem to open up a nearby eluvian and travel through this "in between world" to other mirrors. The gem acted as a beacon, taking them through and simultaneously reactivating several different underground eluvians until they reached the heart of the matrix. There, they encountered a varterral which guarded the center. In this hall were a bunch of eluvians, all connected to certain other mirrors throughout Thedas. Briala then took the gem from Celene and placed into the heart of the eluvian matrix, and therby reawakening all eluvians permanently. When she did so, the magic allowed her to create a passphrase. And with this elven word (which means "Fen'Harel's blessing") any one could potentially manipulate every eluvian. With this word, Briala can open/close, deactivate/reactivate every mirror in Thedas. And what's more, she can alter the destination in which she sends someone. With using only one mirror, Briala sent Gaspard and Michel to Val Chevin near Val Royeaux, and then sent Celene through the same mirror to Halamshiral.