The mere anticipation of eating food starts up the digestive process: the brain triggers the release of saliva. The food enters the mouth and is chewed up. Saliva has enzymes in it that digest the simple carbohydrates – so it is possible to completely digest white bread in the mouth ((so it is worth noting that the harder the body has to work to break down the food, the less fattening the food is. It is called the thermogenic effect. This means that the body has to work harder to break down a steak and large salad than it does a tomato sandwich)).
All this chewing triggers anticipatory activity in the stomach. In order to produce HCl (hydrochloric acid)and pepsinogen it starts to release a substance called Gastrin which, by a couple of ways, prods the stomach into producing HCl (hydrocholoric acid) and pepsinogen. ((Pepsinogen, in the presence of HCl converts to pepsin, which is the enzyme that breaks down the proteins into the amino acids and sheers off attached nutrients to enable proper absorption further through the digestive system)). Gastrin also prompts the stomach to start the necessary churning process.
The stomach has a number of regions that the bolus of food passes through as it is becoming chyme. As the chyme reaches the lower parts of the stomach, the level of acidity stimulates three different hormones which variously turn down the secretion of gastrin to lower HCl production; stimulate the pancreas to release digestive enzymes and bicarbonate into the small intestine and stimulate the gall bladder into squirting out bile to digest the fats. In order for this to happen, the chyme has to be properly acidic – a ph lower than 3.
As the clip says, main absorption is in the small intestine. It is the digestive enzymes and bile that enable the food to be further broken down prior to absorption. However food can only be absorbed if it has been properly broken down in the stomach.
The clip finishes by saying nervous stimulation prompts defecation. Since coffee is stimulating to the nervous system, this explains why having a strong cup of coffee can lead to an urgent trot to the loo.