Thanks for the advice. She isn't one of the horses I ride but the lady who does is very quiet and we are training her like a green horse. We are still working on building up muscles to lift her back and bring herself off her forehand. We haven't tried winkers, I'll pass on that tip. We think her reaction is from having the driving whip 'shown' as a cue. It's common and not at all abusive (although we have reason to believe there was more going on than showing the whip) but now she goes forward suddenly when she sees movement behind her. SHe isn't usually ridden when there are others in the ring yet. One of her lessons, though, the owner asked my trainer to get on another horse and ride past at the same gait. To desensitize, you know. They started off passing very far apart and have progressed quite a bit. Now you can pass her with little fuss as long as you give the rider a 'heads up' and pass about five feet away.
~Claire, the frog in the desert