Buffy's face contorted partially from the slow cut. The rest was a look twisted with disbelief and the pain that came from it as the blood surged forward spilling from the wound. She'd not yet lost enough blood to be dizzy, but her head was swimming as she looked up at Lucius in pained shock.
She could feel the power that surged forward to some degree, as she was the source of it, and her soul was tied to a mystical power of some sort that was stronger than she knew. It wasn't right. None of it felt right. She thought to jerk her hand away too late, as she was somewhat weak from either the energies of the ritual or the emotional fatigue of the day's events. As it was, she found that she couldn't muster enough strength to pull it away. It was in her hazy mind, Ambrose's words from earlier rang loud and clear.
Even if he stabbed you, you would still give him the benefit of the doubt, wouldn't you?