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The beginnings of the development of longitudinal balance is in use of up and down transitions: such as walk--trot--walk--canter--halt--walk--trot--collected walk--extended canter--collected trot--(I think you understand the use!??)
Start with ten (10) strides at each gait, then after a few weeks of practice, shorten to 7 strides, after a few weeks, shorten to 5 strides at each gait, then 3 and eventually down to 2 strides. This should take place over a minimum of six months of time in frame of the reduction of strides.
And, always remember, once the horse is showing frustration, or reluctance, then move on to extended movements, performed at only one gait several revolutions of the entire arena, while staying on track. Don't burn out the mind of your horse with these up and down transitions, or you will burn out your horse from wanting to learn and develop more balance, perhaps only with you~!