Question about Blender Cookie's Animation Fundamentals

I’m just going through this series again and I have a question for Beorn (hopefully, he visits this forum from time to time).

Of course, if anyone else can answer, I’d be happy with that, too.

In the video 1.2 Bouncing Ball Exercise starting at 02:16, (Beorn) you say something like: at this point, the ball should actually be over here. Then you move it from above an ‘impact’ point to the top of an arc (impact point and all drawn with the grease pencil).

So, what I’m wondering is: Why should the ball be in the second spot and not the first at that frame? What are you basing that decision on?

It’s not that I don’t see (when watching the final playback of the animation) that it actually does need to move. It’s more that I’d like to know how you came to that conclusion so I have a deeper understanding.