Guys, just let it go. If she wants to try it, she can. If it doesn't work, she can figure out for herself that she's not ready to do this movement. If it does, then hey, she knows how to do it now.
To ask for a flying lead change on a horse who is already trained to do it, you are cantering (lets just say on the left lead so it's less confusing) First you change the bend of your horse using the right leg and the left rein (depending on the horse, you may not need to change the bend) Then you change your position so that your left leg is back, your right leg is at the girth, your weight shifts to the right stirrup, your left hand has as much pressure as your right hand originally had, and vice-versa. Then you swing your hip to the right, and voila. Done. Sometimes it just doesn't work, you have to practice, and be critiqued by a trainer to make sure you really are doing it right, sometimes reading instructions doesn't exactly give you the right idea.
To teach a horse who doesn't know how to do it, this is how I do it. I do it on a figure eight. I go over the figure eight at a trot a few times, just so the horse understands the change in bend. Then I ask for a canter (say a left lead canter while we are on the part of the figure eight that you are tracking left) Before you come to the change in bend, ask for a trot, then change bend, and ask for a canter on the new lead. You basically have to do it over and over, progressing with less steps of trot inbetween the canter. Soon, the horse will understand that change in bend during a canter means to change leads. This can also work on a diagonal, if your horse understands the cues for different leads (eg., right leg back=left lead, left leg back=right lead.) Tell me if it seems confusing, I can put up a video.
Eventing: the only sport where you wear your medical information strapped to your arm.