Her rescues were so successful because she was very strict to make sure no one but those who needed to found out about it. One example commonly noted was that once the group headed on their journey, Harriet Tubman threatened to shoot anyone who wanted to go back, as they could potentially give away which way the group was travelling. She also often used her knowledge of medicine to give screaming children and crying mothers things that would calm them down, to make sure they wouldn't be heard. Hope this helps!