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.
Amos Burn–Erich Cohn1–0D6018th DSB KongressBreslau GER31.07.1912Stockfish 15.1
Queen's Gambit Declined 1.d4 d5 2.c4 e6 3.c3 f6 4.g5 bd7 5.e3 e7 6.f3 0-0 7.d3 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.xc4 a6 9.0-0 c5 10.e2 It's preferable to play 10.a4 hindering black's next move. b5 11.b3 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. b7 12.ad1 12.fd1 b6 13.e5 fe8 14.xd7 xd7 15.xe7 xe7 16.h5 cxd4 17.xd4 with equal chances and
Corsi Ferrari,N (2269)-Lobato,C Mar del Plata 2006 was eventually drawn. 12...a5 12...c7 13.c2 fd8 14.f4 b6 15.e5 ac8 Black's position
is slightly better. Marshall,F-Maroczy, G Ostende 1905 13.e5 Black must
now guard against Nxd7. fe8 13...xe5 loses a piece after 14.dxe5 14.f4 Not at all bad; white is playing for a K-side attack. 14.xd7 was
another option that takes a different path...a center attack, but it fizzles
out. xd7 15.d5 exd5 15...xg5 16.dxe6 favors white. 16.xe7 xe7 17.xd5 f6 18.xb7 xb7 19.f3 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...c7 15.xd7 15.c2 cxd4 16.exd4 h6 17.h4 d5 18.d3 xh4 19.h7+ f8 20.h8+ e7 21.xg7 is favorable to black. 15...xd7 16.d5 c4 17.dxe6 fxe6 18.c2 ad8 19.g4 f8 black has put up a successful defense and
the chances are about equal. 15.c2 f8 The purpose of this move is seen
on move 17, but it was not too late to play 15...Qc7 15...c7 16.f5
is not now playable. xe5 17.dxe5 xe5 18.f4 c5 19.fxe6 fxe6 Black has
won a P and he now stands better. 16.f5 c7 16...exf5 17.xf5 c7 18.xf6 xf6 19.xf6 gxf6 20.g4 e7 21.f1 White stands quite well. 17.fxe6 xe6 18.xf7 Pre-engine analysis aside, this move is quite
correct, so kudos to cautious old Amos Burn! 18.xf6 is also quite
interesting. xf6 19.xf6 gxf6 20.g4 h8 21.xh7 xh7 22.f2 h8 23.d5
The only move that keeps the attack going. e7 24.e4 24.dxe6 ad8 25.xf6+ g7 26.h5+ h6 and the chances are equal. 24...g7 25.d6 d8 26.exf6 with complications, but white is a bit better. 18...xf7 By
accepting the sacrifice black exposes himself to withering attack. Instead, he
sould have looked around for counterplay. 18...xg5 19.xg5 d6 A
fascinating position! White has two plausible continuations that leave him
with a slight advantage. 20.d5 20.xh7 xh7 21.d5 xh2+ 22.h1 xd5 23.xh7+ xh7 24.h5+ g8 25.xd5+ h8 26.f3 g3 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.g5 e6 28.xg3 with the better game. 20...ad8 21.f5 c8 22.e6+ xe6 23.xe6 e7 24.xd8 xd8 25.g3 White is better. 18...b4 19.a4 19.xf6 xf6 20.h5 f8 21.xf6 gxf6 22.h6+ h8 23.d5 xd5 24.xd5 g6 with an unclear position. 19...xf7 20.h5+ 20.xf6 xf6 21.h5+ e7 The K is slipping away to safety. 20...f8 21.xf6 xf6 22.d5 d8 23.d6 f7 24.xh7 c6 While black is not out of the woods, he has
avoided to worst. 19.h5+ g8 This allows white to gain a tempo which is
decisive, but even the better 19...Kf8 was not likely to save him. 19...f8 20.xh7 xg5 21.xg5 ad8 22.e4 White has a decisive attack because the
threat of e5 cannot be met. 20.xf6 White is clearly winning. xf6 21.xh7+ f8 22.d5 e5 22...g5 23.xf6+ 23.dxe6 xe3+ 24.h1 xe6 25.g6 Black resigned because the threat of Rxf6+ is too much. 25.g6 ed8 26.de1 g8 26...d7 27.h8# 27.xf6+ gxf6 28.e7# 1–0
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