I can show someone exactly how a movement should look, clean and textbook. But the moment they're actually doing it wrong in front of me, I freeze, unsure what to say to fix it without confusing them. Demonstrating I've got; catching and correcting their form as it happens still slips right past me.
More people experience this than they realize.
You can demonstrate a move but can't fix someone's form as it happens.
If this sounds familiar, the Library can help you find the bigger picture.