I think it might be something like in Death Note. The Death Note only works on a person if it's physically possible for the written details to take place. IE, a criminal in jail could be written in the book as escaping from jail, fleeing to Paris, and then dying of a heart attack on the Eiffle tower. That couldn't possibly happen, so the criminal would simply die of a heart attack where he was. In the sense of Geass, it might be the physical impossibility restraining the girl from completing her task.
This concept might even relate to how Euphie reacted to the Geass. Like Lelouch mentioned, it was practically impossible in her mind to ever do something as terrible as massacering hundreds of Japanese people, so her nature reaction was to fight the power.
![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
ninetydegrees![[community profile]](https://www.dreamwidth.org/img/silk/identity/community.png){kind=small}
(no subject)
This concept might even relate to how Euphie reacted to the Geass. Like Lelouch mentioned, it was practically impossible in her mind to ever do something as terrible as massacering hundreds of Japanese people, so her nature reaction was to fight the power.