That is certainly what it used to mean in traditional Western horsemanship. However, I understand the term now includes kinder (and ultimately more effective) progressive desensitization, where the scary stimulus is repeatedly applied and removed again before the horse reacts enough to want to escape. It requires the handler to be more careful, to pay more attention; it emphasizes a conversation with the horse over simply doing a, b and c to the horse in rote fashion. Of course, it's still possible for progressive desensitization to turn into flooding if the horse is restrained so as to prevent escape. However, 'sacking out' can still be done with the horse free to go where it chooses - so it needn't be at all traumatic. The phrase may bring to mind snubbing posts and forcing horses to submit to stuff being done to them, but it doesn't have to be like that nowadays. Maybe it would be better if another phrase was used for the progressive form of desensitization.
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