The resulting increased power and speed of the Bludger can lead to a particularly nasty attack on a player from the opposing team (QA10).
The word "doppel" means 'double' in German. Doppelgängers in fiction and folklore are ghostly - and possibly evil - doubles of a living person. The Dopplebeater Defence is certainly capable of causing harm.
Lexicon lists: Quidditch
From the Web
Harry Potter Wiki: Dopplebeater Defence