"Rolling shutter" is a distortion peculiar to digital video. You read about it all the time in reviews, but it can hard to visualize what exactly the "Jello-like" distortion actual does. Here it is, perfectly illustrated in a short video posted by PetaPixel
There's nothing wrong with the speaker in the video above. The video hasn't been doctored either—it's playing back in real-time. What's happening in the video isn't happening in real life, but it's what the camera sees.
With that image in your head, let's go over what causes the distortion. Basically, when the CMOS sensor in a digital camera is capturing video, it's recording individual frames at a high velocity—let's say 60 frames-per-second. It's not capturing distinct snapshots of time in every frame. Instead, an instant in time is actually captured sequentially across the image sensor over a very short, but not instantaneous period of time. This "rolling shutter" becomes apparent when something inside the frame (or the frame itself) is moving fast enough that you notice the difference in time.
The speaker is playing back a tone with a frequency of 61Hz, which for those of you who don't know your SI units, means 61 cycles-per-second. In theory, to reproduce that tone the speaker would need to vibrate at precisely that frequency.
Meanwhile, the camera (a Canon 6D) is recording video at 60 frames-per-second, and each frame is captured at a virtually instantaneous 1/4000 of a second. But because the frame rate and the vibration frequency are ever so slightly out of phase, the distortion is especially apparent as it ripples through the image.
Usually, rolling shutter doesn't look this intense, but this video does a nice job illustrating why it drives videographers nuts. [PetaPixel]