Sometimes a whip-like hand motion is added for emphasis.
2. Can be used to replace the expression "oh snap!" such as when a major victory/butt kicking takes place.
3. If someone 'burns' another person verbally, saying 'wha-chh' afterwards is also appropriate.
"Yeah, his girl's got him tied around her finger."
2. "We totally KILLED that team from the other school at last night's game! WHA-CHH!"
"Yeah, those pansies!"
3. "You smell like farts!"
"Haha! Oh, WHA-CHH!"