Waiting for the horse to resist is actually too late.
Trainers can read a horse’s mind the same way an adult can read a 5 year old child’s mind. You reward when the horse is thinking the correct thing. If you reward too late the horse is then thinking of resisting and you reward when the horse is thinking of resisting before it actually resists.
I have not seen it addressed here, however over time and observing students, there IS a difference with riders who grew up riding and those who started later in life.
There is a connection with horses that seem to be missing with the latter.
My instructor brought it up with me when seeing me groom -something happens, horse is upset, groomer knows and just waits for thinking to change and continues grooming. Just subtle and most people would not have even noticed. Horse never changed just horse thinking changed.
Both our partners in life who have been taught by us, and other students do not have this ability and we have been unable to teach it.