1. What does it mean?
Short for 억지 텐션, 억텐 describes energy someone deliberately puts on, such as exaggerated laughter or excitement. It may describe an effort to keep a conversation or a show lively.
Also written or said as 억지 텐션, eokten
2. The nuance
It can be a self-aware joke about performing cheerfulness. Directed at somebody else, it may dismiss their reaction as fake; you cannot reliably know their feelings just from a clip.
3. Examples
“너무 피곤해서 오늘 방송은 억텐 좀 섞였어.”
I was so tired that some of my energy on today's stream was forced.
“내 농담에 억텐으로 웃어 줄 필요는 없어.”
You don't have to force a big laugh at my joke.
Sources
Nate News: Korean usage in the press
References support the meaning and usage. Example sentences are written for NoCapWiki.