I used to say it. I also used to believe it.
"My horse keeps dropping their shoulder around the barrel."
I simply didn't know any better, and believe me - bought many a bit that promised to fix this pesky problem.
That was until I learned the horse's shoulder is attached to their body by nothing more than muscle, tendon, and ligament. They have no collar bone like humans do.
When their front leg is touching the ground, it acts like a pillar, and it is impossible for it to drop.
But riders aren't wrong in the fact they are feeling... something. What they are feeling is actually the horse's ribcage dropping, or rotating.
I could go on, but there are many other pros who can describe it better than myself, so if you want to take five minutes to unlearn the old and relearn this foundational information - check out this excellent video by Wendy Murdoch - complete with a horse skeleton demo!