Difficult part about defense is evaluating when groups should be defended. Often there are sequences that are so-called "rightful sente" for the player that has them. For example, the opponent can rightfully peep a between one-point jump and the defender should connect. Pre-emptive defense here is not to connect those two stones before peep, but some other move altogether - if it's even needed.
From attacker's point of view, an attack is success when it forces opponent to live small and gives the attacker some kind of benefit (points of territory or thickness). But from defender's point of view preventing that successful attack is not necessarily good defense!
One very simple example of what I'm talking about:
- Click Here To Show Diagram Code
[go]$$W White plays a rightful sente
$$ ---------------------------------------
$$ | . . . . 2 1 O . . . . . . . . . . . . |
$$ | X X X X X X O . . . . . . . . . . . . |
$$ | . O O O O O . O . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]
- Click Here To Show Diagram Code
[go]$$B Black plays poor 1 point gote
$$ ---------------------------------------
$$ | . . . . . 1 O . . . . . . . . . . . . |
$$ | X X X X X X O . . . . . . . . . . . . |
$$ | . O O O O O . O . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]
Defense is something that makes eye space or eye shape. In this purpose it prevents attack:
- Click Here To Show Diagram Code
[go]$$B 2 defends, 3 attacks
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X X O . a . . 2 . 3 . . . . . . |
$$ | . . X , O . O . . , . . . . . X . . . |
$$ | . . . . . . . . . . b . . . . . . . . |
$$ | . . 1 . . . . . . . . . . . . . . . . |[/go]
In above diagram white should defend invasion of "a" by jumping at "b". Was this joseki choice poor for white, because he has to defend?
