There are many who think that diaphragmatic breathing might help in stuttering. The main proponent of  it is the McGuire's programme.

We did some web-surfing and found an excellant how to on diaphragmatic breathing, by Swami Jnaneshvara Bharati. Here are a few excerpts -

"The diaphragm is a huge muscle that rests horizontally across the base of the rib cage. Imagine an oval shaped dinner plate or bowl, turned upside down, and inside your lower rib cage. The diaphragm is connected in the front, along the sides of your lower ribs, and also along the back.

On inhalation, the diaphragm muscle contracts, and pulls downward, such that the ribs flare out slightly, and pulls the bottom of the lungs downward to bring in air. On exhalation, this releases and the air goes out. With the Yoga practice of deep diaphragmatic breathing, the space just below the breast bone, at the upper abdomen pushes in slightly so as to exhale more completely.

When the diaphragm muscle contracts, it pulls the bottom of the lungs downward, causing them to fill, while the ribs flare outward to the sides. The chest and abdominal muscles are not used in diaphragmatic breathing. Conscious diaphragmatic breathing is extremely relaxing to the autonomic nervous system and is essential preparation for deep meditation.

When the diaphragm is used for breathing, there is no motion in the lower abdomen, and the upper chest remains still. However, many of us lead stress-filled lives, and learn bad breathing habits, using the abdomen and the chest. This creates further tension that leaves us in a vicious cycle of mental chatter driving bad breathing and physical tightness, and the bad breathing, in turn, causing trouble to the mind. While there is no motion of the upper chest and lower abdomen in proper diaphragmatic breathing, it may take some time and practice to attain this motionlessness, and to have the motion occur only in the diaphragm area itself."

The original article can be found here. This page has some illustrations, and a youtube video, a must-see.