Find the best move for black. it's easy but it's an amazing move.That's why I share it. Its from a real game.

It is a nice move.  Promoting to a bishop would also work.

Technically, it's not just a discovered check... it's a double check.  The only way to get out of a double check is to move the king, if possible.  That's not possible here, so it's mate.

Yeah, I'd go for a bishop promotion myself. You can never have too many underpromotion checkmates.

Except promoting to a bishop checkmates one move later (1... f1=B+ 2. Kf3 Bce2#) so it isn't really optimal...

However, as is well known, the formula for promotion aesthetics is 100/(NP) where:

N = the number of moves to checkmate

P = the relative value of the piece created by the promotion

In this case, the Q promotion with a mate in 1 results in an aesthetics value of 100/(1x9) = 11.1.  A promotion to a B with a mate in 2, on the other hand yields a promotion aesthetics of 100/(2x3) = 16.7, a slightly higher value.

Obviously the only move was the best!

Yeah, right....