Time Reversion
From Code Lyoko Wiki
Time reversion, or, as Jeremie calls it, "return to the past now!", is one of the greatest capabilites of the supercomputer. It allows the user to return to a point in time of their choosing. Just how far back it can go is unknown, but it's usually limited to roughly a day. For some unknown reason, those who die cannot be brought back by a return trip. In addition, each return trip doubles the power of the supercomputer, which in turn doubles XANA's potential power, thus making a theory that since Franz Hopper used a time reversion so many times, XANA gained so much more power then the programing he was designed to be, and as such gained sentience and took advantage of this and began ploting to destroy humanity.Team Lyoko usually activates this after an attack to make sure nobody remembers the events that transpired. All people with Lyoko ID Cards usually remember the events of the previous timeline, though there have been exceptions, such as Odd in Killer Music and Attack of the Zombies.
As shown in Amnesia, time reversions can and do regain the memories of people, but only if a amnesia pandemic caused by XANA has spread.
The characters in the show have used the feature to their own personal gain. Ulrich used it to win the lottery in a misguided attempt to prevent Yumi from moving back to Japan, which got him banished from the group temporarily. Franz Hopper used this feature 2546 times over the course of his research, clearly becoming more and more mentally unstable with each trip. It can also be assumed that Jeremie is becoming a little crazy as well, as he gets into hateful arguements with lots of people, especially Aelita, his lover, but only mostly in Temptation, In which doesn't seem to cause him any trouble with any other Time Reversions.
Jeremie is usually the one that activates the time reversion process. All others that do such without his say-so are punished by immediate banishment from Team Lyoko, like Ulrich's banishment in The Chips Are Down.
In the third season, Jeremie seemed more relaxed about returning to the past even though sometimes it wasn't needed like in the first season.
