Something about that scene that I've always wondered about. There were six vampires there in the basement, not counting Spike. Sarah had been shown handling that many at one time before, and she was doing fairly well here until two, and ONLY two, vampires grabbed her arms. Then she looked at Spike and just quit struggling. Even when she thought that he was going to bite, and almost certainly kill, her, all that she did was just grit her teeth and flinch. Then, as soon as she realized that he actually wasn't going to kill her, she actually looks surprised before breaking away from the two holding her and then she takes out all six vampires like it was nothing.
To me, it always just looked like she almost WANTED Spike to kill her, or that the shock of it being him just froze her momentarily, neither one very typical Buffy.