White’s knight masks his queen, which otherwise could take the loose rook on a7. The only question is what White’s knight might do while unmasking the attack. It can’t give check, but might it again add to a threat against a square next to Black's king that already is under attack? Yes: White’s queen is aimed at g7, and the knight can add more pressure to the square with Ne6. Now White has Qxg7# if Black tries to save his rook, so Black instead has to fend off the mating threat and let the rook go on the next move, losing the exchange. White's 1. Ne6 is as good as a check.