she can only pull it out of your hand if she gets her neck braced and away from you at a large enough angle. what I mean, is if her face is kept bent around to facing you more, and she tries to barge off away from you, youi'll be able to put a good firm pull on the rope, and she'll have no choice but to disengage your hind and she'll be facing you.
Mac does this sometimes, especially when I try to give him a bath. he can barge the rope right out of my hands, and no amount of strength can stop him, even in a rope halter (which I recommend you use).If he gets his head facing AWAY from me, gets real stiff in his neck and then can pull away from me so fast it's amazing.
the trick is to never allow him to tip his nose more than 45 degrees off from directly facing me. he can still walk forward, around me or with me as I lead him, but if I allow him to straighten out his neck, I am toast.
so, keep a bend in her neck, nose tipped well toward you.
that's kind of a short cut, though, since teaching her correct leading will be your real goal.