Simple: Ask for more speed.
1) Ask her to back up like normal. Then, give her an extra amount of pressure a click your tongue to ask for more speed. If she doesn't, warn her by adding another dose of pressure. Then, if she still doesn't add speed make her think she is chicken little and the sky is falling xDD
Ask, Tell, Demand. This is how we train horses. Just remember the minute you get what you want release and praise her. :)
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.