Keep the flex in her neck (or pressure on the bit) until she stops moving her feet and relaxes. Then release the rein pressure immediately as her "reward" for doing it right. Even if she walks around for 15 seconds, if you keep the pressure on the bit, she will realize that what she is doing is not the right answer. She may try other things such as backing up, throwing her head, etc. but do not release until you get the correct response. She will quickly realize what you are asking and respond with the right answer.