Why threating a mate here loses a pawn?

There is a similar pitfall in the Sveshnikov Sicilian 1 e4 c5 2 Nf3 Nc6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 e5 6 Nbd5 d6 7 Nd5 Nxd5 8 exd5 Ne7 9 c4 a6 10 Qa4 Bd7 11 Nxd6#

Before Nb5: White has an excellent position, mainly due to the space advantage.
After Nb5: Black must play Ng6 to stop the mate, White must exchange pawns on e6 or lose material. White then has to play Qc2 to avoid losing material. The result is that white lost his space advantage, must retreat his advanced pieces, and black has a center majority.

I'm be a little surprised if the computer didn't question Qa4+ as it just helps black develop.

Stockfish assumes play as good as it can think of. For this reason, traps or tricks that give a slightly worse position are considered to be bad moves. Good traps and tricks don't come with strings attached.


Wow very precise answer thank you! I just saw the stockfish evaluation dropped 1 point after Nb5 , and that means it is one pawn loss somewhere but i did not know where. Thanks!

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