From Traderpedia

The nickname given by locals to a trader, Paul Rotter, allegedly because of his unusual trading style involving rapidly flipping orders for thousands of contracts from one side of the bid over to the ask in the Bund futures.

Paul Rotter was propelled to fame when traders discovered he was personally responsible for 10% of all volume in the Eurex German Bund future - the worlds 2nd largest futures market after the Eurodollar. Accused of market manipulation and an unusual trading style flipping huge orders for thousands of contracts across the bid/ask, the nickname "the Flipper" was coined and Paul was made the bogeyman for every Bund traders losing trades.

Rotter however, in his interview with Trader Monthly magazine refuted these claims and says that he is just another local in the Bund who's been there for over ten years and isnt doing anything that he hasnt been doing trading for a long time.