1. What does it mean?
유동 shortens 유동닉 and is often contrasted with 고닉. On forums such as DC Inside it commonly refers to posting without the persistent registered identity used by fixed-name members.
Also written or said as yudong
2. The nuance
This is community jargon, not a guarantee of technical anonymity. Two posts with the same visible anonymous name need not be by the same person, and site-specific systems vary.
3. Examples
“유동으로 쓴 질문이라도 답변은 친절하게 달아 줬어.”
They answered kindly even though the question came from a non-fixed-name poster.
“같은 유동 이름이라고 전부 한 사람 글은 아니야.”
Posts under the same anonymous name aren't necessarily all by one person.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.