I’m afraid this is not a groovy 1960’s reference to the Gunny Saks dress designer. But you’re sort of close. It does involve using scary things that are frequently white and floppy. Since horses are prey & flight animals, their first response to anything scary is to unload whatever is up there (you) and RUN for their lives.
“Sacking Out” is the process of desensitizing the horse to the kazillion possible things that frighten them. It’s a long process, and important. For example: police horses need to learn not to run from the sound of gunfire. I run at the sound of gunfire, and I’m (technically) a predator. Catching the drift? Roping horses have to get comfortable with something swinging wildly overhead making helicopter noises. (As well as loud rock music, I think I lost an ear drum at the last event.)
Often the scariest thing a horse encounters in its life time is the white plastic grocery bag. Even if it was just full of carrots that are now safely deposited in the carrot bank: their stomach. Horses natural predators are lions and wolves…white bellies…so anything white and wafting, WATCH OUT.
The term “Sacking Out” comes from the days when cowboys used old flour or burlap sacks to flap around the horse until the horse realized nothing bad was going to happen. Nowadays it’s likely to be western horse blankets, tarps, old jackets, and grocery bags that get flapped, but it’s the same idea. Of course they used other things (ropes, etc) but the “Sacking Out” is still what people commonly call this phase of training. Which, BTW, never actually stops. No matter WHAT that trainer promised you for your 90 days.
Some examples of Sacking Out: