1. What does it mean?
총공 shortens 총공격, all-out attack. In fan communities it can mean acting together at an agreed time, such as promoting a release or taking part in a voting campaign. Elsewhere it may describe a coordinated wave of criticism.
Also written or said as chonggong
2. The nuance
The word alone does not tell you whether an action is benign. Read the actual request, respect platform rules, and do not confuse collective support with permission to harass people.
3. Examples
“팬들이 공개 일정에 맞춰 응원 총공을 준비했어.”
The fans prepared a coordinated support campaign for the announced release time.
“총공이라는 말만 보고 참여하지 말고 내용을 읽어 봐.”
Don't join just because it says 'group campaign'; read what it actually involves.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.