YouTuber mrbibio calls the optical illusion you see here "the static flow of water." The description, of course, is an oxymoron; how can something that is static also be flowing? And yet, the water in this video — which you can hear trickling, and surely must be flowing — appears to be suspended in midair. So what's really going on here?

mrbibio explains:

You're actually seeing 25 different drops [or segments of the stream] every second... the waveform being played through the speaker is vibrating the pipe in a uniform way, so the drops exit the pipe uniformly, the frame rate of the camera is synced with the speaker so the drops appear static, [and] the short shutter speed means the drops appear sharp and frozen in mid air [a longer shutter speed, in contrast, would capture more of each droplet's movement, causing it to appear blurry].

In other words, the camera is shooting at 25 frames per second, which is exactly the same rate that the tube carrying the water is being vibrated by the speaker. Furthermore, the stream of water exiting the tube not only breaks apart into individual droplets, it actually does so in a repeating pattern. As a result, the droplets that form assume a nearly identical shape (and occupy an almost identical point in space) every 1/25th of a second. A slight hiccup in the droplets between 1:49 and 1:56 reveals what happens when the framerate at which the camera is filming falls out of sync with the frequency of the repeating droplet pattern.

A similar effect can be achieved with strobe lighting. The difference is that, with a strobe, you're matching periods of illumination (rather than the framerate of a camera) with the frequency of your droplet pattern. The number of times the strobe flashes per second, and the duration of each flash, correspond to the camera's framerate and shutter speed, respectively.
[mrbibio via reddit]