She didn't react instantly, but she does have heightened reactions. She has time to react as he's rolling, and that reaction time wouldn't matter much because she'd be stuck with her downwards swing in the time of her reaction anyways, and once she see's him roll out of sight then her sword would probably be done with it's swing and she could start her next swing. Although this is only if I keep her doing this move when I edit the post. I guess she'd still do it rather then just try to leap away or swing her tail around wildly at him. She would unlikely lift her left foot up if she can't see him behind her, although if she tries to spin to her left then she's probably use her right foot instead to lift up and spin her around. I did the spin move in person and that's the most comfortable way.