Smothered mate is a chess term used to describe a type of checkmate. Smothered mate occurs when a king is completely surrounded by his own pieces and is put into check by a knight. If the knight cannot be captured, the king is put into checkmate as a result, as it has nowhere to move. SOmetimes though, the king doesn't need to be checked by a knight; as a queen could work almost as well. It just needs to have one square on a line with the king open, and all the other squares are blocked off. Such is the case with a back rank mate in the corner.