Desensatization :) I usually have someone hold the horse (or vice versa) and take a soft cotton rope (in case you accidently do hit it or flip it around on them) and start swinging it slowly. If it backs up, follow it. If it sidesteps, follow go with it. Continue swinging the rope. As soon as the animal settles and is disinterested, stop and praise him. Do this in several sessions, eventually getting to where you can flick it between his legs, under his belly, over his head, by his face, and above his head. I did this with my mare and she doesn't even flinch when I whirl it by her.