|notyetagm: The following position is from this game, which occurs at White's 27th move:|
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White is threatened with the mating 27 ... ♘f3+ 28 ♔h1 ♕xh2#. White's problem is that his pieces are unable to partake in the defense of their king. How poorly is the White king defended? He cannot even defend the f3-square to keep the Black e5-knight out! White has terrible light-squared weakness around his king but his light-squared bishop is over on b3.
The only way to meet the mate threat is to vacate the f2-square to give his king a flight square, by playing 27 f3 or 27 f4. 27 f3 drops the f-pawn to 27 ... ♘xf3+, leaving White with a weak e4-pawn and giving Black a beautiful central outpost on e5. Thus 27 f4 seems preferrable, vacating the f2-square to avoid the immediate mate threat but retaining the f-pawn to avoid the positional weaknesses that result from its loss.
Unfortunately 27 f4?, played by Seirawan in the game, is a blunder that allows a beautiful mate. Polgar responded with 27 ... ♘f3+ 28 ♔f2 ♕xh2+! and White resigned because he cannot avoid being mated.
If 29 ♔f1, then 29 ... ♗h3# is an obvious mate. But why not play 29 ♔xf3? Well then in that case Polgar plays the wicked <decoy> 29 ... ♗g4+!, dragging the White king up the board and into a mating net with 30 ♔xg4 ♕h5#.
Now we see the important difference between 27 f3 and 27 f4?. 27 f4? retains the f-pawn but it <self-blocks> the critical f4-square, allowing Black to trap the White king in a mating net with the diabolical <decoy> 29 ... ♗g4+!. If White had played the correct 27 f3, then after 27 ... ♘xf3+ there would be no White f-pawn and hence the f4-square would not be self-blocked so there is no mating net!
Chess is such an unforgiving game. Play 27 f2-f3 and you survive; play 27 f2-f4? and you're mated.