Ok Important thing to note, at the beginning of the fight he pulled the chain on his device. And whenever he goes back in time he always goes back to that 1 specific point in time. From what I note, he can't just hit a button (or wrist) and go back in time, he has to set a beginning check point to he will always return to.
I think the ult will the same, you have to use forethought and plan where ur check point will be. And I think that an ult that requires skill like that is something that Riot could do. R to set the checkpoint, and R again to go back about 4 sec, he might be able to do this multiple times, but probably at the cost of a longer CD, or more mana. Im also betting that its effect is limited to a certain radius. And as we all notes it will only reverse his location, the enemies, but its not gonna negate damage or death.
He also seems to be pretty fast, so that may be a part of something.