Why do people talk into their fists when excited or surprised?

Has anyone ever noticed that a lot of people, mostly American, and mainly African americans talk into their fist when they are surprised or shocked? I was wondering what the origins of this were. If anyone has any idea id appreciate some constructive answers, please abstain from posting racist or demeaning comments! If you look at any video from "pimp my ride" where an african american person has their car "pimped" the reaction is always the same, the fist held to the mouth almost in a "microphone holding" pose. Any ideas?