Chess in password game

https://preview.redd.it/u4p9r9u2jegd1.png?width=486&format=png&auto=webp&s=5f6c10a8323205176be68545cd84a71d179820b5

So I was playing when I came across rule 16, the chess, and I can't understand waht I need to do or what is the answer. somebody please help.