Someone who is on the ball is intelligent and able to do a good job.

一个人on the ball，那么这个人就很聪明，能做好工作。

Your feet are in a similar position. All your weight is on the balls of your feet.

脚姿势相似。重心也都在你的脚上。但这两个表达有不同的目的与含义。

" Somebody is on the ball" . Can you imagine...? Okay, here's a ball and somebody's standing on it. It's very difficult to stand on the ball. All right? So you have to work hard.

“有人在接球” 。 你可以想象. . .？ 好的，这是一个球， 有人站在上面。 站在球上非常困难。 好的？ 所以你必须努力工作。