Even though two knights cannot force checkmate (with the help of their king) against a lone king (with the exception of positions where White wins in one move), decreasing the material advantage and allowing the defending king to have a pawn can actually allow for a forced checkmate. The reason that checkmate can be forced is that the pawn gives the defender a piece to move and deprives him of a stalemate defense . Another reason is that the pawn can block its own king's path without necessarily moving (e.g. Kling & Horwitz position right).