I think what he’s getting at is that there’s a limit to how far you can, for example, pull an arm if you’re grabbing it through chainmail. At some point you reach the limit of how far the chainmail will move, and then the mail itself prevents you from pulling any farther, so you wouldn’t be able to pull the arm off without first tearing the chainmail.
I suppose you could let go, pull the mail up a bit, and re-grab the arm if the person inside didn’t draw the arm back in quickly, like if they were unconscious.