Today's subtlety in defence

This is all part of the 5-second tactics series.
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SUBTLETY IN DEFENCE
fm Kostin, Konstantin
Position before Black’s 20th move. Tchigorin Memorial, last week. White has carefully aligned his pieces with intention to proceed with a Kingside attack. Especially, his Bishops have taken up the classical diagonals and his Rook on the 4th rank is very mobile…HOWEVER, the Black monarch is well entrenched , without pawn weakness. The Knight on d5,in particular, guarantees that being Black will not easily be over run.
In this kind of situation nerves can play a decisive role for the defence . Often it is necessary to be cold blooded and not be easily frightened by the enemy: you have to rely on cold, accurate calculation. Here Black correctly decides that he must simplify:
20…Nd7!?
If White intends to attack, he must strike very quickly, especially as Black might soon play …f7-f5, neutralizing the diagonal b1-h7, a familiar defensive theme in such positions.
21.Nxd7!

(21.Bxh7+ and only then 22.Nxd7 is the same thing)
HOW TO RECAPTURE THE KNIGHT?
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Analysis shows that the ONLY correct defence is to recapture with the Queen:
21…QxN!
Now routine attacking methods will fail: 22.Qh5?! f5! or 22.Rg4 g6! followed by …f5. INSTEAD, White must play more energetically:
22.Bxh7+! KxB 23.Qh3+ Kg8 24.Bxg7!
This looks immediately decisive and threatens mate in one! The reader can easily verify that taking the Bishop loses very quickly after 25.Rg4+
Most players , on arriving at this position in their analysis would not look any further and instead go back and look for something earlier. HOWEVER, it is precisely in this kind of situation it is necessary to be cold-blooded and dig in and search for hidden resources. If your position is sound then there must always be a resource! Lasker wrote of the necessity of believing in your position and having faith in your valuations:
”And if you have searched in vain a hundred times, continue… your valuations may be at fault: prove them and improve them. But, first of all, search diligently; work, for such work is rewarded.”
24…f5!!

Remarkably, this move –as ugly and improbable as it may seem–may hold the game! Taking the Rook on f8 actually gives Black the advantage, as does 25.Be5? Bf6!, the reader can easily verify for himself. It seems that White has nothing better than (with the help of a chess engine to check the tactics) :
25.Qh8+! Kf7 26.Qh7!
26…Nf6! 27.Bxf6+ Kxf6 28.Nf3 Bd6 29.Qh4+ Kf7 30.Rd1 Rg8 31.Rcd4 Qe7 32.Rxd6 Qxh4 33.Nxh4 Rg4 34.R6d4 Rxd4 35.Rxd4 Rc8

And while no one should doubt that White is better (he is a pawn up) Black has excellent counterplay and real chances to hold the game! It is not easy for White to coordinate his pieces. IN ANY CASE, had I had the attacking position that White had before, I would be very reluctant to settle on this ending!
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GETTING BACK TO THE GAME:

Black erred in the defence and took the Knight with his Bishop…
21…BxN?
This natural move overlooks a subtlety in the attack
22.Bxh7+! KxB
Now 23.Qh3+ (as in the previous analysis) would fail after 23…Kg8 24.Bxg7 KxB 25.Rg4+ Bg5! 26.f4?! e5! winning
23.Qh5+! Kg8 24.Bxg7! f5

25.Be5!!
It is forced mate!
Black resigned after 25…Rf6 26.Rh4.
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