Rule of 11

In bridge, the rule of 11 is applied when the opening lead is the fourth best from the defender's suit. By subtracting the rank of the card led from 11, a defender can figure out how many cards are higher than the card partner has led. The rule works because the defender is known to have three and exactly three cards higher than the card led.

When West leads the Spades7 East applies the rule of 11. This shows that there are 4 cards higher than the Spades7 that dummy, declarer and East holds.East can see four cards higher than the 7: The SpadesQ8 on the board and the SpadesAT in his hand. Knowing that declarer does not have any cards above the Spades7 (As out of the 4 cards east, dummy and declarer have, all 4 are in possession of declarer and east himself, leaving none for the declarer), if the dummy covers with the 8, East knows he can win the trick cheaply with the 10.
However, the declarer can apply the rule of 11, too.When West leads the Spades7 South can see the four higher cards in his hand and on the board. Accordingly he knows that the play of the 8 from the board will win the trick cheaply.
Since the rule of 11 helps the declarer more often than the defenders some people have begun experimenting with leads other than the fourth-best. Journalist Leads generally call for the lead of the third or fifth best (showing count) while others propose leading small from a suit the defender would like to see returned and high from a suit the defender doesn't want returned (showing attitude).