Diaphragm separates the chest cavity from the abdominal. The diaphragm- is a dome-shaped wall, consisting of several muscle and beams layers, which subtracting, participate in the act of breathing. There are two anatomical openings in the diaphragm. One, the fibrous ring, is for the blood vessels. It is quite rigid and does not vary in diameter to protect the blood vessels from damage. Other, esophageal aperture, is formed by muscles. It may expand and narrow depending on the food bolus.
Disorders in normal functioning of the backbone lead to internal organs disorders. So scoliosis (spinal curvature), which most of the people now have to lesser or greater extent, leads, on the one hand, to the disorders in normal organs innervations through the spinal nerves, on the other hand, to the internal organs displacement in their respective cavities of the body. It results in reduction of their tone and function.
The same happens with the diaphragm: its tone is reduced, the dome sinks, while the above-mentioned esophageal opening is increased in diameter, much more than necessary. Part of the stomach or even the whole stomach gets from the chest cavity into the abdominal cavity through the above mentioned widened esophageal opening when the pressure increases in the abdominal cavity or the body gets horizontal position.
This is hiatal hernia. Please, read about hiatal hernia symptoms.