Well, it's used in singing to control the outflow of breath (and to help stay in tune). So in speech, it can help regulate the flow of breath so it's smooth and not choppy. Also, focusing on the diaphragm takes your attention away from the throat, lips and tongue where you may experience the physical aspects of a speech block. It can increase awareness of when you are running out of breath and need to breathe in, which also gives you a little pause to get the next few words clear in your mind. I'm sure someone could offer a more detailed, even scientific, duscussion.