I don't like backing horses out of trailers, my horse chiro said it is better to walk them out and I notice it's less stressful on the horse, no doubt you noticed that too, huh?
Hmm, that may be the case with a nice wide stock crate, but generally IME chiro's would get heaps more business having horses getting jammed turning in the float!
Agree with Cherie, that even if it wasn't a necessity, I'd teach it anyway. It's just one of those basic things - I teach them to go forward, backwards, right, left and over stuff on cue, because well, even if you can't put a purpose to it when you're training, one day out on the trail you may find you need those specific lessons known.
I would teach her with pateince & consideration that she is frightened about it though(not at all saying you don't BTW Cherie, but 'giving one chance' sounds a bit abrupt IMO).
I do it in a similar manner to what Cherie describes, but if there are steps involved, I do what you did Tully & start by getting them confident doing it outside the trailer first, and when in the trailer, I give them a number of goes practicing half in, half out before getting them all the way in. I also don't generally have the luxury of a helper, so in order to get them to back out & not turn, I have them on a halter with very long lead, looped through a tie ring at the front of the float. Then I can just let it slide without pressure when they're doing Right, but put pressure on & block them when they try to turn. With only one of us, I don't have the liberty of pulling a horse out, but having put the preparation in, I don't find they take long to work it out.