Arresto Momentum.

MomentUM.

It occurred to Olly that his problem with the spell the first time he tried it was that he had said it wrong. He paused for a few moments, thinking carefully about what he had said just minutes earlier. That was very likely his problem.

"Arresto MomentUM." He said it aloud once without waving his wand, just to get used to it. He had to be able to do it right every time if he wanted to win. Slytherin was already far behind. But there seemed to be less of them, which was completely unfair in his opinion. Why did they HAVE to do the spell anyway? Couldn't they just catch the balls? That would be a lot easier. But it was CHARMS class and not catching class, so of course the answer could not be as simply as that. Fine. Olly turned his attention upward to find another target.

"Arresto Momentum." He pointed his wand at a falling ball and waved it furiously. Perhaps, too much? Considering nothing happened, he had done something wrong. "Arresto Momentum," he said again. This time his wand-waving was calmer. More decisive. And that seemed to work. The ball slowed, and Olly slam-dunked it into the Slytherin bin.