When exposing a horse, I do things until they are no longer "novel." Novelty to a horse is something to be afraid of or to defend themselves from, it unconsciously ignites the fight/flight response in a horse.
A mistake I often see folks make, is biting off more than the horse can "chew," and the horse tips into a reactive state of mind instead of a learning/processing state of mind. Or another mistake; they stop the stimulus or exposure to the novel concept or object too soon, and instead of the horse re calibrating how it thinks about that thing, their negative reaction or behavior becomes worse because they now associate that "thing" with high emotion or danger.
To help the horse with things that are novel, whether they are concepts, maneuvers, or objects, I expose them a piece at a time, until there is no more worry or concern at that stage, amount of time, or distance in association to that "thing."
In this photo, I'm exposing this horse to the novel concept of tossing that rope, up and over the top of his head...something I know is harmless, but he thinks is dangerous. Repetition of the motion, in a deliberate and incrementally increasing fashion, is what helped bring this horse from fearful to confident and understood.