I've heard so many times that it involves desensitizing the horse to things by scaring them with it until they just don't care anymore...and I really wouldn't want that for my horse. That's not trust.
Trust is bonding with your horse to the point that if a "scary" plastic bag floats by his face in the wind, he'll look to your calm reaction for reassurance as to how HE should react. Honestly, that's what my horse and I have. I never waved things in front of his face or threw things around or what have you. We have a special bond and we trust each other. If something typically frightening to a horse happened, he would look to me for guidance as opposed to going off his rocker. A horse shouldn't be trained out of its natural instinct, it should learn how to trust its partner.
But that's just my two cents...or maybe I just spend too much time with my horse.
And a side note - I always think of something my farrier said whenever people talk about how they want their horses to live completely natural. "If you want your horse to live 100% naturally don't shoe them, don't blanket them, don't halter them, don't feed them, and take down your fences so they can roam free...because that is what's natural." I think it's a little harsh, but it gets the point across to some of the more ridiculous people.