It involves an increase in motility of the colon consisting primarily of giant migrating contractions, in response to stretch in the stomach following ingestion and byproducts of digestion entering the small intestine.
[1] The reflex propels existing intestinal contents through the digestive system helps make way for ingested food, and is responsible for the urge to defecate following a meal.
[1][3] Clinically, the gastrocolic reflex has been implicated in pathogenesis of irritable bowel syndrome (IBS): the very act of eating or drinking can provoke an overreaction of the gastrocolic response in some patients with IBS due to their heightened visceral sensitivity, and this can lead to abdominal pain and distension, flatulence, and diarrhea.
[4][1] The gastrocolic reflex has also been implicated in pathogenesis of functional constipation, where patients with spinal cord injury and diabetics with gastroparesis secondary to diabetic neuropathy have an increased colonic transit time.
Since the reflex is most active in the mornings and immediately after meals, consumption of stimulant laxatives, such as sennosides and bisacodyl, during these times will augment the reflex and help increase colonic contractions and therefore defecation.