A volvulus is a twisting of an organ that causes its blood supply to get cut off. A very simple but effective mental picture would be a balloon animal- as a loop is twisted to stay in place the base of it becomes very thin and decompressed. While many organs can get a volvulus (stomach, small intestine) the most common presentation of volvulus is of the large intestine (colon). Patients will have symptoms of bloating, very localized pain, inability to pass gas or have a bowel movement.
This occurs because the colon is attached to fixed tissue that contains its blood supply. The colon can move around this fixed point which tethers it to the back of the abdominal wall. Over time the colon can become stretched out due to age, medications, chronic constipation. Some people are just born with extra long colons. The volvulus occurs when the colon twists around its blood supply, creating a loop that prevents the colon from functioning.
Once the loop is formed the blood supply can be cut off. The tissue starts to suffer from lack of oxygen, potentially leading to necrosis and perforation. A diagnosis is usually made with an x ray or CT scan. Attempts to untwist the loop can be made with a colonoscopy. Surgery is the next option. The loop of bowel may require resection to prevent this from happening again.