If your horse has a good whoa when walking forward, give her those cues when asking her to flex.
With moving her hip, say you are flexing her neck to the left. You would bring your rein back towards your pocket (not up by your shoulder, or down by your knee), and give her the cue to give. If she starts circling, you would rub your heel on her left side, a few inches behind the girl so that she would disengage her hindquarters and step to the right. (or outside of your 'circle')
Does that make any sense?