peristalsis

Peristalsis is the traveling wave of contractions in the muscular wall of the digestive tube, and it is what moves food and then waste along. The wave squeezes the tube just behind its contents and relaxes it in front, so the swallowed material is pushed onward in one direction — esophagus, stomach, small intestine, large intestine, rectum, anus — and normally not back the way it came. That one direction is why the tube is called a one-way street, and why an item that reverses it is a mistake rather than a detail. Peristalsis is not digestion and not absorption: it moves material, while the small intestine's villi absorb the products and the large intestine reclaims the water that is left. The wave is also what carries food down the esophagus against gravity, and what moves the material through a tube the body cannot tip.