Imagine watching a video and, just as somebody drops their phone, a dozen comments shoot across the screen. They seem to be reacting together. Have you accidentally joined a live stream?
Probably not. On Bilibili, a Chinese video platform, these moving comments are called 弹幕 (dànmù). Bilibili describes them as flying comments. They appear over the picture, linking what somebody has written to a particular moment rather than leaving the whole discussion in a list underneath.
That small change gives a comment something it usually lacks: timing. A reaction can arrive alongside the expression, mistake or tiny background detail that prompted it, so you don’t have to work out which part of a ten-minute video the writer meant.
The same moment, different days
Imagine someone leaving a comment on Monday and you pressing play on Friday. Their reaction can still appear when the awkward dance move happens. You aren’t necessarily sharing a live conversation, but the video brings you back to the same point in it. Researchers studying this kind of comment describe the feeling as co-watching: the sense that other viewers are there with you.
An ordinary comment saying “wait for it” is rather unhelpful if you read it before starting the video. Placed just before a surprise, it becomes part of the build-up. A joke can depend on arriving at precisely the right second; move it elsewhere and it might make no sense at all.
The same thing could help with a detail you’ve missed. Picture a scene in which a character walks past a poster that becomes important later. A comment at that moment might make you look twice. Someone else’s attention briefly changes your own, without requiring you to stop watching and search through a separate discussion.
Of course, that also explains why people might prefer a clear screen. The helpful hint could give away an ending. A flood of reactions could cover the poster altogether, and a rude remark remains rude even when it has excellent timing. A comment layer adds another voice to the scene; it doesn’t automatically improve it.
When the picture isn’t accessible
There is another question beyond whether the comments look fun. If you can’t easily see the video or read words moving over it, how do you follow a conversation that relies so much on the screen?
A 2025 research paper called DanmuA11y, by Shuchang Xu and colleagues, examines this problem for blind and low-vision viewers. It identifies difficulties including missing visual context, competing speech and a discussion that is hard to organise. Simply reading every comment aloud would not necessarily solve those problems.
Think of our imaginary poster again. Hearing “look at that!” tells you very little if nobody explains what “that” is. And if an audio version of a comment speaks over a character’s crucial line, gaining the comment could mean losing the story. Making more information available and making it understandable are different tasks.
The researchers developed a system that turns time-linked comments into audio discussions with additional context. They evaluated it with 12 blind and low-vision viewers. That is a specific research project, rather than evidence that every video platform has already introduced the solution, but it shows how much thought a seemingly simple feature can require.
A crowd you can join differently
You might want other people’s reactions during a comedy and prefer silence during a complicated scene. Someone else might need the conversation presented in another form entirely. Those choices aren’t a test of whether you understand the platform; they are different ways of enjoying a video.
The next time a scene makes you want to turn to a friend and say, “Did you see that?”, the appeal of flying comments is easy to understand. Their clever trick is to keep that reaction attached to the moment, ready for another viewer who hasn’t arrived yet.
Explore another screen format and a different way of sharing a fandom: TV drama for an upright screen and What is guzi, and why do people collect it?.
Sources
- Bilibili: International FAQ: What is danmu?
- Xu and colleagues: DanmuA11y: research on time-synchronised on-screen comments