How does Pat Parelli's Friendly Game address desensitization and what are the key principles?

Pat Parelli's Friendly Game is the first of his Seven Games specifically because without it, none of the other games can be played safely or effectively. A horse that does not trust the human cannot learn from the human, and the Friendly Game is the systematic process of building that trust through non-threatening contact and progressive exposure. The core principle of the Friendly Game is simple: the handler keeps doing something near or on the horse until the horse shows relaxation, then does more of it — more area of the body, more intensity of movement, more duration. The two critical rules are that the handler never retreats when the horse is afraid (which teaches the horse that fear removes the stimulus) and never increases intensity until the horse has relaxed at the current intensity. Parelli teaches the Friendly Game with a training stick and string, rubbing the stick on the horse's body, swinging the string over the horse, throwing the string over the horse's back, and gradually moving to more intensive stimuli as the horse accepts each level. He is specific that the movement must continue without break until the horse relaxes — stopping when the horse is still afraid teaches the horse that staying afraid makes the stimulus stop. The Friendly Game progresses from the handler's hands to the training stick to tarps, ropes, bags, flags, slickers, and eventually to objects specific to the horse's work environment — saddles, roping dummies, cattle, vehicles. Each new stimulus is introduced with the same approach-and-reward pattern, and Parelli teaches that each successful desensitization generalizes slightly to future stimuli, so that a horse with extensive Friendly Game experience in many contexts becomes progressively easier to desensitize to new things. Parelli's overarching point about the Friendly Game is that it is never finished. Throughout a horse's life, new environments, new stimuli, and new stressors mean the Friendly Game should be revisited and updated regularly. A horse desensitized at home may need the Friendly Game played again at a new show environment, on a trail, or after time off.