The number of times a square is attacked and defended cannot be properly calculated by simply counting the number of pieces controlling that square. Rather,

<<THE NUMBER OF TIMES A SQUARE IS ATTACKED AND DEFENDED CAN BE COMPUTED -ONLY- AFTER -ALL- THE -THREATS- IN THE POSITION HAVE BEEN TAKEN INTO ACCOUNT.>>

Part I (++). Being able to <-MAKE-> a threat (especially <CHECK> or a <MATE THREAT>) means that you control a square <-more-> times than meets the eye (because you can <MOVE WITH TEMPO (FOR FREE)>).

Part II (--). Having to <-MEET-> a threat means that you control a square <-fewer-> times than meets the eye (<OBLIGATION RESTRICTS MOBILITY>).

Maybe I can add attackers to the square with tempo, making a threat.

Maybe you can add defenders to the square with tempo, making a threat.

Maybe some of my attackers are tied down meeting threats.

Maybe some of your defenders are tied down meeting threats.

---

Spassky vs Averkin, 1973

White to play: 26 ?

click for larger view

The beginner: White's 26 Bc7 is indeed a nice move. Not easy to see, but makes perfekt sence after all. The constalation of Rook, and Queen on same diagonal, should always ask the question is there someway to exploit it. Here spassky use the bishop as decoy :-) would make a nice puzzle

Position after 26 f4-c7!

click for larger view

The underlying factor which makes this excellent tactical blow possible by Spassky (White) is that there is <LOOSE SQUARE NEAR THE ENEMY KING>. The White g3-rook attacks the g7-square, and this square is <DEFENDED> only by the Black g7-king. So this <LOOSE> g7-square next to the Black g7-king makes an excellent <TACTICAL TARGET> for a <DOUBLE ATTACK/QUEEN FORK>.

Spassky saw that if he played e2-e5, he would do so <WITH TEMPO> since he now threatend the <SNAP MATE> e2xg7#. <<<So -every- square that can be reached from the <TACTICAL BASE> the e5-square would make an excellent second <TACTICAL TARGET>>>>. This second square that can be threatened by the White queen from the e5-square is the c7-pinning square.

So after 26 ... c8xc7 27 e2-e5

click for larger view

Black has no time to meet the threat of 28 e5xc7 because he must deal with the immediate mate threat 28 e2xg7#. So Spassky wins the exchange ( for ).

RECAP: In the original diagram, Spassky saw that he could move his White e2-queen to the e5-square <FOR FREE (WITH TEMPO)> by threatening mate on g7. Hence his queen is effectively -already- on the e5-square, meaning that the c7-pinning square is not 1-1 loose but rather 2-1 en prise.