Are you a tennis fan? If so please explain

[Roscoe]

I was watching the tennis today while waiting for a visitor to arrive and I noticed after each volley they looked down to the ground and shook their fist and I wondered why they did that. I thought that it may be some sort of tennis ritual but not being a tennis fan I have no idea why they do it, but they do it repeatedly and I wondered if someone could tell me what it was all about.