When you promote you can promote to ANY piece on the game. But they are YOUR pieces to move. You win by capturing the king(s) of your opposing color. So when you checkmate you have to take an extra move to eat the king. Lets say you are white and you decide to promote to a BLACK queen. This queen cannot capture any of its black counterparts it can capture yours though but you get to control it so you can use it to help create a stalemate etcetera. Tell me if you like this variation I certainly do!
I'd be glad to try to understand and memorize these rules -- if I could be sure it wouldn't make my head explode. (Sorry -- being old, am getting short on neurons -- it's a miracle I can still remember en passant...)
this would be checkmate