It's true that your body, along with the baby's just \"knows\" when it's time to be born. The hormone levels change in the uterus which sends signals to the rest of your body to get ready for the big day.
A friend of mine has tied the Castor Oil and it did bring on labor for her. It also \"cleaned her out\".
These things work on some people because it artificially stimulates your womb to contract, like the Braxton Hicks, mentioned earlier. It's cramping and cramping, which not only helps to push out a baby, but it also pushes everything else out.
If you're not ready, you're just not ready.