Here's a more detailed explanation:

... Bxf4

(There are a couple things white could do here, including taking the pawn at g6+, but nothing that really puts white ahead, because black is eventually looking to promote the pawn for discovered check. White can't take the pawn with his rook before it's promoted because he loses the rook. He can't take the promoted pawn either because he needs to get out of check from the black rook. Uncovering the king by taking the bishop with his pawn would work to white's disadvantage against a rook-queen attack.)

Then white's only move would be:

Nf2

Then black could either take the white rook with the new queen or try to maneuver into mate with the rook and queen.