Chess Online

I agree with the portion of this exercise that the player is expected to solve, but how in the world is the move 11. Rxa6 the best move for white? It leads to mate in 1 while there are many moves that do not lead to mate in 1. Is it a mistake? Maybe just the computer's way of resigning the game?

Read the continuation of the line. At move 11, white makes the move Rxa6 (I didn't pull that out of nowhere).

This is what the board looks like before Rxa6:



So you can see that moving the rook anywhere along the a-file leads to 11. ... Qb1#, while doing, say, 11. Rg1 delays a mate. Yet in the continuation the computer chooses an inferior move leading to immediate mate.