"It happened when he was cursed in the Forbidden Forest," Enola blurted out suddenly, taking over from Alice. "Try to understand, Hermione, Killing Curses leave a hell of an impression when they hit their victims, and this one struck Harry directly in the face. Tom Riddle was very angry with Harry when he tried to kill him that night five years ago. The curse reflects that, so try not to react too badly when you see it. Most people are shocked to their foundations on first glance, so Harry keeps his face hidden beneath shawls and scarves almost all of the time. It's a reflexive response and Harry understands that ... sort of. But that sound - of a disgusted gasp of shocked horror - is a sure-fire way to make Harry shut down in front of your eyes. I've seen it happen. It takes a little bit of light from his face each time it does."