Re: Scratch that, reverse it Edge of Tomorrow...
Would you believe I am actually watching that movie right now? Waiting for some data copies to finish I popped it on and,in checking I logged in here and . . .
I saw the movie once before and assumed that the 'reset' was an involuntary reaction triggered upon the death of an 'Alpha'. Once Maverick unwittingly vampires an Alpha, his death provokes the same response - a reset. The response is not deliberate as the 'Omega' is an largely an unthinking being.
The way I saw it was that Captain Couch Jump aquired the traits of the creatures whose blood he vampired. The 'reset the day' action can be seen as an arbitrary use of a general power which, in the case of the aliens is applied in limited form to save an Alpha. It must be involuntary, else why reset a whole day? Why not just the last 10 minutes?
So, Ethan Hunt isn't actually resetting the day - he is dying which the unthinking, reactionary 'Omega' is see as the death of an Alpha and thus is resetting the day. When he get;s to the end of the movie, he absorbed the Omega's blood and thus the Omega's powers to reset time. And so, un wittingly, he resets it but, as he is human with different emotions than the Omega, he winds it back to a different point.
In short, the death of someone with Alpha blood causes the the Omega to reset time involuntarily. Charlie Babbitt acquires the Omegas ability buy, as a human (aren't we great?!) he doesn't have the same restrictions.