"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 :)
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.