This project is a simulation of the result when filming a propeller with a camera with rolling shutter, such as in a mobile phone.
If you are confused by this, check out this video: https://scratch.mit.edu/discuss/youtube/dNVtMmLlnoE/ Thanks to @MonkeyBean2 for giving me this idea. Thanks to @njdavison for the rolling shutter line. Don't forget to check out @MonkeyBean2's project, which is based on the same idea, but is generated using nothing but math: https://scratch.mit.edu/projects/600923234/