I have taught my young horse to back up when asked from the ground. I have transferred this to doing it under saddle by applying the aids for rein back, and using the same word I used during his ground work. It took him three attempts to get it, and he will now rein back from the aids alone under saddle. He is, however, super sharp and very very intelligent, so it may not be so easy to do with a slower reacting horse.
Worth a try though, it was a stress-free way of training him.
My RI builds a little box with poles leant on barrels, you ride the horse in and the only way out is to backwards, with rider using pressure from each leg one at a time, and a gentle pull with the corresponding hand to encourage the movement.
I disagree with what you say about teaching a slower reacting horse Maiko
Missy is very laid back poster below you will vouch for that
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She needs alot of leg , but when reining back I like you ...... have used the same aids as I would have used on the ground . I basically have to apply squeezing of my leg and say the word "back" and away she goes .
Op when teaching your youngster to rein back , make sure you ask them to go forward straight away