Disentangling himself from his robes he got quickly to his feet, wand up, but Sirius said, 'Petrificus Totalus! ' and Snape keeled over again, rigid as a board.
Harry saw his scared white face look down at the ground falling away, saw him gasp, slip sideways off the broom and — WHAM — a thud and a nasty crack and Neville lay facedown on the grass in a heap.