Speak From Your Gut
When people say "Speak from the gut" or "Trust your gut," they are advising their listener to trust their instinct or "inner voice." There really is a lot of wisdom in those words, in many ways.
First, your power source for speech, air, is most effective when you allow it to sink deep down into your body. When you inhale and exhale, it is your abdomen that is moving out and in; your shoulders shouldn't be rising, and your chest shouldn't be getting real big. When you allow breath to drop down into your body, you are also helping to ground yourself.
When sound comes from your gut, it will have depth and resonance. If you think of sound as coming from your throat or mouth, your voice will not have the qualities that contribute to a strong voice-- one that can fill a room with your presence. Groundedness, being relaxed, and using all of your body for resonance will translate into a vocal quality of confidence and authority. People who don't take advantage of their full "instrument" when speaking often sound as if they are "holding back," unsure, or even weak.
Breathing down into your belly helps you draw a deeper connection to your "gut instinct." Did you know that the gut contains 100 million neurons, and has been called a "second brain" by researchers? It contains a complex network of neurons, neurotransmitters, and proteins that respond to stress and pleasure, much like the brain in your skull. It is the only part of the peripheral nervous system that can elect not to do the bidding of the brain or spinal cord. Your gut literally has a mind of its own.
Maybe that's why people who suffer from terrible anxiety or stress sometimes develop ulcers and other digestive problems. Maybe they're not "listening to their gut" as much as they should.
Also, if you are familiar with some forms of meditation or bodywork practices, you may be aware of the "hara," which is located just below your navel and a few inches in. It is often described as the center of balance, as well as the "vital spirit." Meditational practice incorporating the hara involves focusing attention on that area during breathing (the power of your speech) exercises. If you're not familiar with it, look into it; it's good stuff.
So, you really should speak from the gut. You'll sound better, and chances are you'll feel better, as well.