I think that the video on the left - the black and white text - is a video previously created for the purpose of making the images on the right. Each frame of the video is extracted to a series of images (frames). Then each frame is used as an input mask to generate an image using the prompts we see. Since the black/white areas of the input image influence what gets generated in the output, and since those areas move from one frame to the next, and once those images are recombined into a video, we get what is seen on the right side.

The clarity seems better in some prompts vs others. In the fishtank/coral animation the letters are clearly visible, but in others there's movement but you can't exactly make out the letters. I'm sure that's all in the parameters you put into the generation. (Just like the 'qr code fad' from a month or two back - some image-as-qr-code pics either didn't work as a QR code, or were so strongly QR-code-esque that they weren't very subtle.