Nd7 not only attacks the queen but threatens to win a full rook by discovered check with Nf8+ and Nxe6+.
Okay, had to solve it first. Rook checks so this is a forcing move. What better moves are there in principle? Knight attacks Queen with a tempo of development into a better position. The development threatens a fork on the rook and King protecting by rook on back rank. But here's the bit which may be confusing, it appears now that you can't save the rook without being in a worse position (zugzwang). So black gives up the rook. Even if black kept the rook there and places the queen on e7, Knight to f8 is still crushing as black king would be forced to back rank, and taking the rook with a second check. So to save the position in the most depressing way, the line black chose is less volatile in the long run.
This topic has been archived and can no longer be replied to.