could someone explain to me how sated devourer works...

Once a yi gets his sated every other auto attack is treated like two. So say his next auto proced his passive and sated at the same time. He would hit you for a total of three attacks, two from his passive plus one more for his next auto attack. The reason why it is so strong on him is because it reduces the amount of time it takes for his passive to go off and and he deals more true damage from Wuju Style because as I said before, it applies on hit effects.