You can’t undo the same 24 hours twice.
Probablity is truely random, I call this the “quantum coinflip”.
If you activate it on 12PM on Wednesday, the world reconstruct itself and the events from 12PM on Tuesday up to the moment of your ability’s activation.
The old timeline ceases to exist, all living organisms consciousness immediately shifts into the new timeline, they do not remember the previous timelines.
If you then activate it again at 1PM on the same Wednesday, you can only change events from 12PM to 1PM (for only for the elapsed hour, since the preceeding 24 hours before that are already write-protected). This second activation would mean that there’s another “quantum coinflip” to determine the outcome.
Any event past 24 hours is unchangeable, it’s permanent.
Once someone is dead, they cannot be saved. Something with 0% chance of happening cannot happen. Something with 100% chance of happening will always happen. Regardless of which timeline.
However, someone very close to death but not dead, you can dramatically increase their chance of survival if you land in a good timeline.
Conversely, someone getting an easily survivable gunshot would become near fatal if you roll into the bad timeline.
“Good” and “Bad” are subjective to your PoV and moral code. For example, if you think that a mass murderer should die, in your “good timeline”, they’ll be more likely to die, and their would-be victims would be more likely to survive, but remember there’s also the 50% chance there would be more deaths if you land on a bad timeline.
[And before anyone gets too smart and find loopholes:
For this hypothetical: A second is defined as: “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom”, a minute is 60 seconds, an hour is 60 minutes, don’t use alien clocks lol, there’s no tricks to bypass this.]
Assuming I’m relatively aligned, I think I proc this more or less daily until I either have high risk of death, or things have improved substantially. Each day I should include some random event, like a dice roll or a draw of an MTG hand, so I can measure if the most recent reroll was a + or a -. Also, definitely start living extremely safely.
Treat base-timeline as 0, and one good proc as +1 (one bad as -1). Then this is a random walk on the integers, memoryless (markov) and balanced (equal odds of going in each direction). This random process tends to end up sqrt(N) away from 0 after N steps are made, revisits 0 infinitely often, and by symmetry is equally likely to be positive or negative. So with daily procs for 50 years, you have a reasonably high chance of reaching +138. So stopping at +80 or +100 feels both achievable and likely incredibly beneficial no matter what the implementation looks like (though you’ll probably spend years at -60 or worse; so safety really matters!).