It took years for the medical community to finally learn what causes a side stitch. Suddenly a runner develops pain in the right upper part of the belly, just underneath the ribs in the front. With each step the pain worsens. Doctors proposed all sorts of explanations for side stitch and most were nonsense. A stitch is not caused by gas in the colon because it is not relived by passing gas. It is not caused by a liver swollen with blood during running, because the liver has a very distensible capsule and does not enlarge much during exercise. It is not caused by cramps in the belly muscles because the belly muscles are not held rigidly when you have a side stitch, and it does not hurt when you push on the belly muscles. Lack of oxygen to the diaphragm doesn't cause them because blood flow to the diaphragm is not shut off by running. They are not caused by trapped gas in the lungs because gas does not get trapped in the lungs during exercise.
The first reasonable explanation and successful treatment came from Dr. Tim Noakes. Thick fibrous bands called ligaments extend downward from your diaphragm to hold your liver in place. When you run, your liver drops at the exact time that your diaphragm goes up, stretching the ligaments and causing pain. Humans have a fixed pattern of breathing when they run. They have a two to one breathing ratio, breathing once for each two strides. Most people breathe out when the right foot strikes the ground. When you breathe out, your diaphragm goes up, and at the same time, the force of your foot strike causes your liver to go down. This stretches the ligaments that attach the liver to your diaphragm, causing pain. So the cause of a side stitch during hard running is a stretching of the ligaments that hold the liver to the diaphragm and the cure is to relieve the stretching of the ligaments.
When you get a side stitch, stop running and press your hand deep into your liver to raise it up against your diaphragm. At the same time, purse your lips and blow out against the tightly held lips as hard as you can. Pushing the liver up stops stretching the ligaments. Breathing out hard empties your lungs. Usually the pain is relieved immediately and you can resume running as soon as the pain disappears.