"Sacking Out" It's to desensitize to flapping, noisy things, etc. Helps teach them that the scary horse eating monsters of the world aren't so scary with a leader (handler) to help them through it. I use tarps a lot. All over their bodies, on a pvc frame to make a tunnel with a low ceiling, walking over them, etc. My youngest stud drags one around the arena and shakes it & plays with it a lot. I like that he sacks himself out! Lol!
I have a couple of example situations where that training paid off. As a kid I rode my mare for a lot of rodeo & show openings. I was carrying in the american flag, the wind caught it just right and wrapped it around her head. She had enough trust that I could guide her safely and didn't take a misstep.
On a different horse we were warming up in the outdoor arena outside of the coliseum at our state fairgrounds. A coliseum employee was hanging out the window beating the dirt out of rugs. Lots of antsy horses over that one but mine wasn't one :)
It's very scary. Noise, texture, appearance... All unfamiliar and predatory in a horse's mind. I use tarps in case I ever have to drap anything across my horse's back (coats and such), wear one of those oversized ponchos while riding, or have her step across something like that (like when crossing barbed wire with a coat over it). I also don't own a rain sheet and put a tarp across my horse when I'm outside in the rain waiting for judges.