On the ball Meaning   to be alert, focused and able to reac | English Choice

On the ball

Meaning
  to be alert, focused and able to react to something quickly.

For example

 My assistant is amazing. He's always on the ball noticing mistakes and offering valuable suggestions.  

Although he doesn't have a medical condition he uses the same prescription medication that people who have ADHD use to stay on the ball during exams. @engchoice