There are two ways to do outcome pumps, one simple and one very tricky, which you might choose between when defining the rules of your universe’s magic system. Which one’s better depends on what your readers like, and how much trickiness you yourself can handle as an author. The simple way to define it is to say that when you cast a spell, universes that descend from the one in which you cast it are reweighted, but their total weight is preserved. The tricky way would be that the weights are not conserved, which would allow limited backwards causality. For an extreme example, consider Nullus Universus, a spell that sets the weights of all universes in which it’s used to zero. It would be impossible to observe such a spell being cast—people would mysteriously decide not to use it, or mispronounce the words, or whatever was the most-probable branch in which it isn’t used—but precommiting to cast it if something else happened, would prevent that thing from happening. (Such a spell would almost certainly be lost quickly, though!)
There are two ways to do outcome pumps, one simple and one very tricky, which you might choose between when defining the rules of your universe’s magic system. Which one’s better depends on what your readers like, and how much trickiness you yourself can handle as an author. The simple way to define it is to say that when you cast a spell, universes that descend from the one in which you cast it are reweighted, but their total weight is preserved. The tricky way would be that the weights are not conserved, which would allow limited backwards causality. For an extreme example, consider Nullus Universus, a spell that sets the weights of all universes in which it’s used to zero. It would be impossible to observe such a spell being cast—people would mysteriously decide not to use it, or mispronounce the words, or whatever was the most-probable branch in which it isn’t used—but precommiting to cast it if something else happened, would prevent that thing from happening. (Such a spell would almost certainly be lost quickly, though!)
They already have time travel.