The phenomenon is called the ideomotor effect and you can witness it yourself if you hang a small weight like a button or a ring from a string (ideally more than a foot long). Hold the end of the string with your arm out in front of you, so the weight hangs down freely. Try to hold your arm completely still. The weight will start to swing clockwise or anticlockwise in small circles. Do not start this motion yourself. Instead, just ask yourself a question – any question – and say that the weight will swing clockwise to answer "Yes" and anticlockwise for "No". Hold this thought in mind, and soon, even though you are trying not to make any motion, the weight will start to swing in answer to your question.

Magic? Only the ordinary everyday magic of consciousness. There's no supernatural force at work, just tiny movements you are making without realising. The string allows these movements to be exaggerated, the inertia of the weight allows them to be conserved and built on until they form a regular swinging motion. The effect is known as Chevreul's Pendulum, after the 19th Century French scientist who investigated it.

What is happening with Chevreul's Pendulum is that you are witnessing a movement (of the weight) without "owning" that movement as being caused by you. The same basic phenomenon underlies dowsing – where small movements of the hands cause the dowsing wand to swing wildly – or the Ouija board, where multiple people hold a cup and it seems to move of its own accord to answer questions by spelling out letters.

If you attach your wedding ring on a string and hover it over the reclined, pregnant brides tummy, it will tell you if it's a boy or girl. :-)  In theory.....

http://community.babycenter.com/post/a24946545/wedding_ring_gender_test..was_it_accurate