“What is magic?
Then there is the witches' explanation, which comes in two forms, depending on the age of the witch. Older witches hardly put words to it at all, but may suspect in their hearts that the universe really doesn't know what the hell is going on and consists of a zillion trillion billion possibilities, and could become any one of them if a trained mind rigid with quantum certainty was inserted into the crack and twisted; that, if you really had to make someone's hat explode, all you needed to do was twist into that universe where a large number of hat molecules all decide at the same time to bounce off in different directions.
Younger witches, on the other hand, talk about it all the time and believe it involves crystals, mystic forces, and dancing about without yer drawers on.
Everyone may be right, all at the same time. That's the thing about quantum.”
-Terry Pratchett
That's one way it could work. You see a bunch of possible futures and by knowing them, you are able to force reality into the one that benefits you most. There's probably all kinds of repercussions in the Dresden Files for using this method, but it should work.
Another way would be the simple "glimpse into the future" and by knowing what is going to happen, you can change the outcome. TV shows with that kind of power often depict it as ghosting images of the coming events that overlap with the real time events. So you see your opponent dodging to the right, and instead of swinging left, you swing right. This works perfectly to describe this particular casting stress, as it is simply the pain of being torn between what is and what would have been.
