Rectum

End of the colon where fecal material
accumulates until there is pressure to cause a bowel movement.
The rectum is about 1½ to 2½ inches in diameter and 5 to 7 inches long.
Their are balloon type enema nozzles manufactured that when
inserted into the rectum are filled with air to expand
to fill the rectum so there is no loss of enema solution.
These nozzles can be quite stimulating.

As the enema liquid  and or feces accumulates in the rectum it creates pressure
telling your body that you need to evacuate.