The backward barbs on a cat's tongue make swallowed string hard to spit out, and if one end anchors under the tongue or in the stomach, the gut bunches along it toward perforation.
Linear foreign bodies are surgical emergencies, signaled by persistent vomiting, anorexia, and depression, sometimes with string under the tongue or protruding from the anus.
Never pull string visible at the mouth or anus, since pulling can saw through the bowel wall; go to a vet immediately for removal.
Prevention is storage discipline: sewing thread, ribbon, yarn, earbuds, and wand-toy cords go away after use, and string toys get supervised play only.
