1. What does it mean?
광대승천 describes an uncontrollably happy smile. Here 광대 means cheekbones, not a clown, and 승천 exaggerates how high they rise when you grin at a compliment, a cute moment, or welcome news.
Also written or said as gwangdaeseungcheon
2. The nuance
The expression is playful and visual. It describes delight rather than a literal change in someone's face, and need not imply romantic attraction.
3. Examples
“강아지가 내 무릎에 올라와서 광대승천 중이야.”
The puppy climbed onto my lap and I can't stop grinning.
“합격 문자를 읽는 내내 광대승천했어.”
I was beaming the whole time I read the acceptance message.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.