The idea is, the soft chiming sound made by the bola ball (that is worn on a long necklace so that the bola ball hangs over the bump) can be heard by the growing baby inside the womb. The soothing sound then becomes familiar to them if you wear the bola ball necklace often (which I do!). This soothing sound caused by the bola ball will then also be a comforting and soothing connection for the baby once they are born as, like my very own voice, it is a familiar sound to them.
And I don't see why that shouldn't be true. The same is said for the mothers voice as baby can hear it from about halfway through pregnancy and onwards!
I, of course, often forget that I am wearing it and start to wonder where the chiming sound is coming from until I remember! hehe But I like it! :D