Have you ever noticed how some riders get light, willing forward movement with no visible pressure from their legs, spurs or whips and they have a connected, balanced horse on a loose rein, without having to hold the horse in?
These riders have figured out the psychology of motion!
Whether you are using progressively louder mechanical aids to speed a horse up, or to slow them down, those mechanical aids will become progressively meaningless to the horse due to neural fatigue.
Addressing impulsion through psychology, means being able to bring up a horse life, or dial it down without feeling like you just doubled up on a crossfit class.