It is a Keystroke injecter, means it will act as a keyboard and wite the programmed stuffs very fast. It is done using an external Arduino Microcontroller It can be use to do malicious things like run CMD or terminal and write command very fast, means really fast and upload the payload. Check out this video https://www.youtube.com/watch?v=sbKN8FhGnqg
Check out this video which shows how to use duckey script with arduino https://www.youtube.com/watch?v=ksvo1WDYQ7s
It can be use to spam message as I have shown in this video https://www.instagram.com/p/B4FI2SsnqvC/