Intussusception
A segment of bowel telescopes into the adjacent segment, dragging its mesentery with it, so the resulting venous congestion and ischaemia predictably produce a palpable sausage-shaped mass, colicky episodic pain and redcurrant-jelly stool as the bowel wall breaks down.
First principles
The mechanical problem is one segment of bowel swallowing another
Intussusception occurs when a proximal segment of bowel (most often near the ileocaecal junction) invaginates into the segment immediately distal to it, pulled in like a telescope retracting. This is usually triggered by a lead point (commonly hypertrophied Peyer's patches following a viral illness in infants, or a pathological lead point such as a Meckel's diverticulum or polyp in older children) that the peristaltic wave catches and drags forward, carrying the bowel wall and its mesentery with it.
Educational content pending clinical review. Not medical advice.