notyetagm: 42 ... e4!, a <double attack> which wins. The Black queen gets on the line the White king on h1 and the masking f3-pawn, so one threat is 43 ... f3-f2# discovered checkmate. But when the Black queen went to the e4-square, it <also> threatened the loose bishop on e7. So Black threatens 43 ... f2# and 43 ... xe7 and White has no adequate defense to both of these threats.

The move 42 ... e4! thus sets up the threat of a winning discoverd attack (43 ... f2#) while making another threat (43 ... xe7) at the same time. <Put your line piece on the line of your masking piece and do it with tempo>, that's how the professionals use discoveries.