It means the flush is working. The oil does its job of stimulating the bile, which burns as it comes out along with all the toxins released. If it all were to stay in the body in a longer digestive process, many toxins would be absorbed instead of purged. The highly acidic citrus juice may also burn when it whizzes through. It seems that some people feel such burning, others don't seem to have all those nerve endings in the rectum (lucky!).