The spectators all realized that Bobby was trying to undermine Clara's confidence when he looked at her. Using his spiritual eye, his words were capable of affecting her mental balance. He knew that the strength of her sword of intent depended on her belief in herself, and that half of the battle would be won already if he could weaken her courage before the fight even began. Once her self-esteem was weakened, she wouldn't be able to concentrate enough to use her sword properly.
They both knew that this battle wasn't just about power; there were a lot of other factors involved, such as experience, willpower, and even a little trickery. "I know what you're trying to do," Clara said, breaking out of the spell cast by Bobby's words. "I know how strong my sword is, and I am not afraid of you." She then activated her sword's energy.