When you don't want others in a public restroom to hear your explosive diarrhea. You pucker up and at the flush the toilet and at the same time dump everything you've got while the toilet flush tries to cover your noise. Also gets rid of some lingering diarrhea smells.
Girl 1: Honey I had to dump so bad but the bathroom was full and I didn't want to be embarrassed so I had to pull a Flash Flood Dump.
Girl 2: Dang I know what you mean hun, last time I did that the toilet cut off before I could stop. It was embarrassing. :/