1. What does it mean?
Borrowed from the idea of sudden acceleration, 급발진 can describe a conversation that jumps unexpectedly into anger or a story that races into a dramatic development without enough buildup.
Also written or said as geupbaljin
2. The nuance
Calling someone's response 급발진 can dismiss a legitimate concern. The slang meaning is separate from discussions of actual vehicle acceleration, where the technical details matter.
3. Examples
“가벼운 질문에 내가 급발진한 것 같아서 다시 설명했어.”
I felt I'd overreacted to a simple question, so I explained myself again.
“두 인물이 갑자기 결혼하는 전개는 너무 급발진 아니야?”
Isn't it a bit abrupt for those two characters to suddenly get married?
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.