In order to give you a proper answer to this question, we need to go back to the very base of the matter. FODMAP stands for:
F ermentable
O ligosaccharides
D isaccharides
M onosaccharides
A nd
P olyols

These are the names of a variety of short-chain carbohydrates which are poorly absorbed and rapidly fermented by the gut bacteria, in the gastrointestinal system of people suffering from Irritable Bowel Syndrome (IBS) and / or Small Intestinal Bacterial Overgrowth (SIBO). 

But the question remains, why do these short chain carbohydrates cause so many problems? Well, the first letter of the term FODMAP stands for fermentation and that’s where all hell breaks loose.

Fermentation

‘Bacteria in our colon extract energy by fermenting carbohydrates, but the fermentation process also yields gasses. In the case of glucose the fermentation process leads to hydrogen, carbon dioxide and methane. In addition to accumulating gasses, polyols, monosaccharides, disaccharides and small undigested remnants of oligosaccharides cause what is called an osmotic effect.’*

Osmosis explained

Osmosis refers to the movement of water molecules from a solution with many water molecules, to a solution with little water molecules.

In the digestive system, osmosis occurs as the result of compounds like (1) salt, (2) poorly digested or (3) completely undigested remnants of FODMAP’s.

These three compounds all attract water like a magnet, resulting in a more watery stool than it would have been without the occurrence of osmosis.

This osmotic reaction leads to bloating, gas, diarrhea and pain. Thus osmosis – also known as an osmotic reaction – is an important process to understand, to gain a better understanding of where your symptoms come from. That being said, osmosis is quite shit – literally and figuratively – to experience.