Touch her lightly with the rein, then use your outside leg to "follow through". What I generally do for this is take my spurs off, lay the rein, then kick hard with my outside leg and make the horse spin once, then walk off. I say take my spurs off because I can be a very aggressive rider with my legs so if I'm doing something like that I would rather be kicking with my dull boot and calf than my iron injectors. Eventually the horse learns that if they don't get off that rein, they're going to get kicked over. So they just go wherever then. I mean, don't do this on a green horse, but if your horse is already responsive to leg cues then there isn't too much reason why not.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.