Black attacks the f1 square twice, but the square is protected twice as well by White’s queen and rook. The solution? A simple threat against one of the guards with b7-b5. If the queen captures the b-pawn, it gets taken by the c-pawn; if the queen moves anywhere else along the diagonal leading to f1, it likewise gets taken; if the queen moves off of that diagonal, Black mates on f1. Nor can White's rook be of assistance, since Black mates if the rook moves off the back rank or moves to f1. White's best reply to the pawn thrust probably is Qxb5 followed next move by h2-h3, forfeiting the queen but at least creating a flight square for his king and staving off mate for now. (Or White can start with Qxb5, etc.) The point: pieces that prevent mate, including a back rank mate, are vulnerable. Try forking them; try attacking what they protect; try making threats from squares they guard; try threatening them directly.