>>15
Look at it this way.
On most browsers, you can bring up your browsing history by pressing Control-H. (No, this is not going to become a discussion of werecows.) On Firefox, this brings up a sidebar that shows up on the left side of the window. If you put your mouse over the edge of the sidebar, the cursor will turn into a different kind of arrow. By clicking and dragging it, you can move the edge of the sidebar back and forth. You are, to put it another way, manipulating the border between the normal window and the history window. By moving the mouse, you can increase the portion of the window devoted to either part. In a more extreme view of this situation, you're increasing or decreasing the amount of existence the sidebar has.
Now, let's apply this idea to something more abstract. Look out your window. If you don't live in a highly urbanized area, you should be able to see the horizon. Think of this as the border between the land and the sky. The land and sky are obviously distinguishable thanks to this boundary. Now, if you were to "drag" the sash between the sky and the land, or to manipulate the border between land and sky, you would end up causing the sky to become larger and the land to become smaller, or vice versa. An effect of this might be to cause something that was just on the ground to suddenly be hundreds of feet in the air. Truly a frightening situation to be in. So, look at it this way - manipulating the border between two physical things shifts whatever balance there is in the interaction between those things. Alternatively, by manipulating the border between two things, you can change the manner in which they exist.
Still, this isn't *that* abstract, since it's still dealing with real things in the real world. Many believe that in this world, there are those things that are true, and those that obviously aren't. This divides reality into two extremes: truth and falsehood. But, since we have two extremes, logically one can imagine a boundary between those two extremes - the border between truth and lies. If one were to manipulate this border, suddenly things that were pure fantasy (flying pigs, for the sake of argument) have become reality - or things from reality have ceased to exist. This is how Yukari is said to have invaded the moon - by manipulating the border between truth and lies, as applied to the reflection of the moon on a pond, she was able to make the reflection of the moon into a manifestation of the actual moon, and so send her youkai army onto it. This is what's truly amazing about Yukari's power - the ability to manipulate the border between completely abstract concepts allows her to fundamentally change reality as we know it (at least in terms of two abstract concepts).
Now one of the weird things about Yukari's power is this: is the universe REALLY made of borders and nothing else? Think about it, why exactly are boundaries the thing that binds the universe together? I could make a thesis about how all sorts of esoteric concepts are what make up the universe, ranging from quintessence to belief to dimensions to thought... what makes boundaries so special that they're automatically the one big thing of the cosmos for Yukari? With the correct amount of bullshitting and rules-lawyering, you can quite possibly extend ANY suitably vague and esoteric ability to extend to nigh-omnipotent control over the fabric of the universe. How the hell are you supposed to define boundaries? You could get a council of the world's finest philosophers and theologists and have them debate for ten years straight and they still wouldn't be able to settle on the definitions of the boundaries of the cosmos. Yukari's border manipulation is just plain reality-alteration with an extremely ambiguous and ill-defined method. Furthermore, she would need an level of intelligence far higher than the entire world's IQ combined in order to know how to use her abilities. What she's doing isn't just telling the universe what to do, she's grabbing it by the throat and forcing it to comply to her will.
Now messing with reality is some seriously mind-blowing shit. I still wonder why Yukari hasn't literally divided by zero yet and caused the universe to fold up and implode. Think of it this way: in sci-fi movies you always see ships moving at the speed of light and arriving at their destination without a problem. Would this happen in real life? No, there are an immense number of variables that can occur during the trip such as asteroids and comets and whatnot, and the ship would probably explode by crashing into one of these things. Same goes for Yukari's boundary manipulation. For example, let's say she wanted to let everyone float off the ground by flipping the boundary between repelling and attraction (gravity). So now everyone's flying, but she just fucked with a major law of physics. In a massive chain reaction, nearly EVERY law of physics gets changed due to a shift in the fundamental law of Yukari, so you could have all sorts of unexpected shit going on. Another example: Yukari flips the north and south poles for fun. Guess what? The entire world's magnetic technology comes crashing down thus affecting the entire world's economy, communications are cut off, power goes out, and the migratory patterns of animals get screwed with, all bringing the world down to a new dark age. So Yukari's boundary manipulation still has a fuckton of loopholes, but then the same could be asked about why Sakuya's time stopping doesn't instantly crush her from infinite feedback, how Sikieiki doles out absolute morals in a world of moral relativity, etc.
Seriously now, I can just imagine some poor bastard in the real world asking for Yukari's power, receiving it, and getting absolutely fucked over to the highest degree due to his first boudary switch. And yes, I do know that I'm nitpicking things from a loli shooting game series designed by an alcoholic pedophile. The sad thing is, I don't think ZUN is planning this all out so his explanation of all of this is shoddy, at best.