When you are going down on a particularly fat woman, you wear her fupa like a hat. When she is about to climax you stand up as quickly as possible allowing the fupa to strike the clitoris to achieve an orgasm.
Tim is eating out Penny when she starts to get close, he quickly stands up and her fupa finishes the job. Thus the Fupa Dupa is achieved.
One who has no dupa and a larger than normal fupa. All the fat from the dupa has migrated to the fupa because it has demanded control of the body at this point.