Dumping is a massive rush of fluid into the gut which has gone into a panic caused by the sudden arrival of more sugar than it can deal with. It happens a few minutes after eating (immediate onset), or later after a few hours (late onset). The huge amount of fluid in your gut causes a purge of violent diarrhoea, wind, nausea (sometimes vomiting), dizziness, shakes, hot and cold sweats. It is deeply vile.