on the ball 警觉的；精明的；能干的

【例句】

You need to be on the ball if you want to succeed in this competitive market.
如果你想在激烈的市场竞争中成功，你需要保持警觉。

Make sure you're on the ball during the presentaion—pay attention to every detail.
在演讲过程中一定要保持警觉——关注每一个细节。