Knight delivers checkmate

Tonight I was playing Video Chess and the computer kept using the queen it promoted to attack my king in order to prevent me from getting checkmate when I had black's king cornered before, only I managed to outsmart the computer by taking its queen after it took mine.

I then cornered black's king again after promoting a queen but it was my knight that I used to deliver a checkmate to win! I may have played this move before, only I was playing as black?