To get a horse used to having someone ride it, one technique has been to have someone get on the horse and hold on until the horse stops trying to buck the person due to exhaustion. This technique is referred to as:

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Flooding is a technique where the subject is exposed to a fear-inducing stimulus at full intensity until the fear response is extinguished, similar to holding on until the horse stops bucking.

To Get a Horse Used to Having Someone Ride It