Hm...good question. I think you get to pick the time frame. I think

It'll recreate the event, but the effect/damage will be increased.

It sounds like it repeats what happened before, and if the target was close to death, they will definitely die in the replay.

One of the users is 'Alphamon' which has its own page dedicated to this ability. On it, it says this: "It allows Alphamon to instantaneously replay an elapsed battle so that it can unleash countless attacks in the same instant, making it theoretically impossible to see any of them other than the final blow that brought down its opponent." I think this illustrates the ability pretty well.

So it seems like it essentially redoes the attack over and over again by looping time, but causes the effects of each redo to be retained between loops. So with your example of a nuke, it would essentially be that you nuke them, then turn back time and nuke them again, and turn back time and nuke them again, and so on for as many times as you want, but each time the effects of each previous nuke stack on top of one another. So to an outside observer, they would only see it happen once, but it would have the combined effect of many having been used simultaneously.