Yes, you need to be ready for the leap forward and like Wallaby said, give enough rein so she won't get bopped in the mouth.
If your horse is balking out on the trail, then you might want to improve her responsiveness overall, in an arena where you can more easily deal with her .
She may be less responsive to you all the time than you really recognize.
I know we all accept less from our horses than they could give us if we were willing to demand no less. Well, I know I do. Maybe not others.
So, when things like that start cropping up, it usually means I've gotten really sloppy in what I'll accept in other areas, too.
Time to sharpen up my expectations, and GET what I asked for, and promptly. Often times, then when my horse balks out on the trial, all I have to do is literally rais the crop and he says, "OH, right. I remember, you're serious, aren't you?"