"If the rebels do manage to capture anyone in Bristol, you'll have to be quick," I continue, as if she hadn't said anything at all. "In any case, the prerequisite for trying out the Exit is that the attack goes unnoticed for as long as possible. Or in other words: nobody must escape. Once the Dark Lord learns that Bristol has been attacked, he will attempt to use the Dark Mark to summon those who were there. And if that doesn't work, the next thing he'll do is try to track them down. You don't want him to do that while they're in the trauma room with you."