In addition to the focal-points, other squares in the vicinity of the castled King can also be significant such as weak squares on which the attacker can safely post his pieces.
Text books point out that there are two types of focal points. If the attacker threatens mate or actually delivers mate on a square then it is a mating focal-point, but if he only harasses the King or uses the square as a point from which the launches an attack against the King then it is a strategic or auxiliary focal-point.
There can be more than one mating focal-point which are then referred to as compound focal-points. If there are many focal-points on squares of the same color, iy’s called a weak square complex.
The f7 square is often referred to as an auxiliary focal-point because of its vulnerability before casting. It can also be the focal point of an attack after castling. That said, d7 is rarely a true focal point because after castling mate is rarely delivered on that square. However, it is very often an auxiliary focal-point which is attacked in order to drive the King away or, by means of a sacrifice,
to draw the King on to the mating square.
So much for all the technical mumbo-jumbo. In the following game the cautious Amos Burn sacrifices a N on f7. His mean spirited contemporaries said it was the only sacrifice he ever risked in his whole chess career, but even then it wasn't correct. Actually, the sacrifice WAS sound, PLUS there was also another sacrifice that was equally good. We know this thanks to...Stockfish!
The game was played in the 1912 Congress of the German Chess Federation that was held in Breslau.
[Event "18th DSB Kongress"]
[Site "Breslau GER"]
[Date "1912.07.31"]
[Round "?"]
[White "Amos Burn"]
[Black "Erich Cohn"]
[Result "1-0"]
[ECO "D60"]
[Annotator "Stockfish 15.1"]
[PlyCount "49"]
[EventDate "1912.07.15"]
{Queen's Gambit Declined} 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Nbd7 5. e3 Be7 6.
Nf3 O-O 7. Bd3 dxc4 {Now is the correct time to play this...after white's B
has moved so that he must lose a tempo to recapture.} 8. Bxc4 {[%mdl 32]} a6 9.
O-O c5 10. Qe2 {It's preferable to play 10.a4 hindering black's next move.} b5
11. Bb3 {Better than 11.Bd3. White has the potential to set up a bettery with
the B on c2 and the Q on d3 attacking h7.} Bb7 12. Rad1 (12. Rfd1 Qb6 13. Ne5
Rfe8 14. Nxd7 Nxd7 15. Bxe7 Rxe7 16. Qh5 cxd4 17. Rxd4 {with equal chances and
Corsi Ferrari,N (2269)-Lobato,C Mar del Plata 2006 was eventually drawn.})
12... Qa5 (12... Qc7 13. Bc2 Rfd8 14. Bf4 Qb6 15. Ne5 Rac8 {Black's position
is slightly better. Marshall,F-Maroczy, G Ostende 1905}) 13. Ne5 {Black must
now guard against Nxd7.} Rfe8 (13... Nxe5 {loses a piece after} 14. dxe5) 14.
f4 {Not at all bad; white is playing for a K-side attack.} (14. Nxd7 {was
another option that takes a different path...a center attack, but it fizzles
out.} Nxd7 15. d5 exd5 (15... Bxg5 16. dxe6 {favors white.}) 16. Bxe7 Rxe7 17.
Bxd5 Nf6 18. Bxb7 Rxb7 19. Qf3 {with equality.}) 14... c4 {Cohn was probably
hopingh to push on with Q-side play (perhaps with ...b4), but this move drives
the B to where it ants to ga and black end sup being force on the defenside.} (
14... Qc7 15. Nxd7 (15. Bc2 cxd4 16. exd4 h6 17. Bh4 Nd5 18. Qd3 Bxh4 19. Qh7+
Kf8 20. Qh8+ Ke7 21. Qxg7 {is favorable to black.}) 15... Nxd7 16. d5 c4 17.
dxe6 fxe6 18. Bc2 Rad8 19. Qg4 Nf8 {black has put up a successful defense and
the chances are about equal.}) 15. Bc2 Nf8 {The purpose of this move is seen
on move 17, but it was not too late to play 15...Qc7} (15... Qc7 16. f5 {
is not now playable.} Nxe5 17. dxe5 Qxe5 18. Bf4 Qc5 19. fxe6 fxe6 {Black has
won a P and he now stands better.}) 16. f5 Qc7 (16... exf5 17. Rxf5 Qc7 18.
Bxf6 Bxf6 19. Rxf6 gxf6 20. Ng4 Qe7 21. Rf1 {White stands quite well.}) 17.
fxe6 Nxe6 18. Nxf7 {[%mdl 512] Pre-engine analysis aside, this move is quite
correct, so kudos to cautious old Amos Burn!} (18. Bxf6 {is also quite
interesting.} Bxf6 19. Rxf6 gxf6 20. Ng4 Kh8 21. Bxh7 Kxh7 22. Qf2 Rh8 23. d5 {
The only move that keeps the attack going.} Qe7 24. Ne4 (24. dxe6 Rad8 25.
Nxf6+ Kg7 26. Nh5+ Kh6 {and the chances are equal.}) 24... Kg7 25. d6 Qd8 26.
Nexf6 {with complications, but white is a bit better.}) 18... Kxf7 {By
accepting the sacrifice black exposes himself to withering attack. Instead, he
sould have looked around for counterplay.} (18... Nxg5 19. Nxg5 Bd6 {A
fascinating position! White has two plausible continuations that leave him
with a slight advantage.} 20. d5 (20. Nxh7 Nxh7 21. Nd5 Bxh2+ 22. Kh1 Bxd5 23.
Bxh7+ Kxh7 24. Qh5+ Kg8 25. Qxd5+ Kh8 26. Rf3 Bg3 {White can take a draw with
27.Qh5+ and 28.Qd5+ or he can go for more with 27.Qg5 followed by the capture
of the .} 27. Qg5 Re6 28. Rxg3 {with the better game.}) 20... Rad8 21. Bf5 Bc8
22. Be6+ Bxe6 23. Nxe6 Qe7 24. Nxd8 Qxd8 25. g3 {White is better.}) (18... b4
19. Na4 (19. Bxf6 Bxf6 20. Qh5 Nf8 21. Rxf6 gxf6 22. Nh6+ Kh8 23. Nd5 Bxd5 24.
Qxd5 Ng6 {with an unclear position.}) 19... Kxf7 20. Qh5+ (20. Bxf6 Bxf6 21.
Qh5+ Ke7 {The K is slipping away to safety.}) 20... Kf8 21. Bxf6 Bxf6 22. d5
Nd8 23. d6 Qf7 24. Qxh7 Bc6 {While black is not out of the woods, he has
avoided to worst.}) 19. Qh5+ Kg8 {This allows white to gain a tempo which is
decisive, but even the better 19...Kf8 was not likely to save him.} (19... Kf8
20. Bxh7 Nxg5 21. Qxg5 Rad8 22. e4 {White has a decisive attack because the
threat of e5 cannot be met.}) 20. Bxf6 {White is clearly winning.} Bxf6 21.
Qxh7+ Kf8 22. d5 Qe5 (22... Ng5 23. Rxf6+) 23. dxe6 Qxe3+ 24. Kh1 Qxe6 25. Bg6
{Black resigned because the threat of Rxf6+ is too much.} (25. Bg6 Red8 26.
Rde1 Qg8 (26... Qd7 27. Qh8#) 27. Rxf6+ gxf6 28. Qe7#) 1-0
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