Enchanting the Beaters’ bats makes the Bludgers, which in the 15th century are rocks cut into the shape of balls, even more dangerous. When smashed by the now-magical bats, the Bludgers become flying gravel that pursues the Quidditch players around the pitch (QA6).
From the Web
WizardingWorld.com (Pottermore) features:
Harry Potter Wiki: Quidditch