notyetagm: 13 ♘xd4! ♗x♘ 14 ♘f5! is indeed a wicked <DISCOVERED ATTACK>, creating a <DOUBLE THREAT>.
By moving to f5, the White d4-knight gains control of the sensitive g7-square, allowing White to threaten mate in two with 15 ♕g4+ ♔h8 16 ♕g7#. And by moving off of the d-file, the White d4-knight extends the line of action of the White d1-queen to the <LOOSE> Black d5-bishop, allowing White to threaten to regain his piece with 15 ♗x♗.
Black must meet the mate threat against g7, which he does by moving his king off of the open g-file with 14 ... ♔h8, so that 15 ♕g4 would not come with tempo (no check now).
But then after 15 ♗x♗, White regains material equality while Black has no compensation for his demolished kingside pawn structure and the resulting weak f5-square, which becomes a great outpost for the White knight.