To expand on what sort of trades I mean:
White should take the stone (obviously black extending it in the game was crap)
$$Wc
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . b 2 . X O . . . . . .
$$ | . a 1 X X O . . . X . .
$$ | . 3 X O X . . . . . . .
$$ | . . O O . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$Wc
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . b 2 . X O . . . . . .
$$ | . a 1 X X O . . . X . .
$$ | . 3 X O X . . . . . . .
$$ | . . O O . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .[/go]
Now maybe you are thinking "Oh noes, that was gote! Now he can attack/kill my stop stones. Boo hoo." But the corner is very big, black should atari at
a (or maybe descend at
b but it's rather wimpish and pathetic). So white is quite happy if this happens:
$$Bcm4
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . 2 X . X O . . . . . .
$$ | . . O X X O . 1 . X . .
$$ | . O . O X . . . . . . .
$$ | . . O O . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$Bcm4
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . 2 X . X O . . . . . .
$$ | . . O X X O . 1 . X . .
$$ | . O . O X . . . . . . .
$$ | . . O O . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .[/go]
Now let's say black finishes off the 2 stones. You get a nice big atari in the corner, and as a super magic bonus you have some sort of connection or at least huge yose at
a for later so black didn't really get much at all!
$$Bcm6
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . 2 . a . . . . . . .
$$ | . O X . X O . . . . . .
$$ | . . O X X O . X . X . .
$$ | . O . O X 1 . . . . . .
$$ | . . O O . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$Bcm6
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . 2 . a . . . . . . .
$$ | . O X . X O . . . . . .
$$ | . . O X X O . X . X . .
$$ | . O . O X 1 . . . . . .
$$ | . . O O . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .[/go]
So let's say black descends in the corner (he could instead block and then there is a big ko fight). White is then happy to have got the corner move in sente to make that group have good eyeshape and can tenuki. At some point white can consider pulling out the two stones. But the big difference now is that after those moves in the corner not only is white running with a weak group, but the black corner is not alive either so there are chances to counter attack. Just for example say black goes tally ho attacking and neglects his corner. The hane at e18 his a key point that makes black have a false eye in the corner, builds white's eyeshape and aims at jumping to
a next, whilst black's group gently dies. Because white has played those moves on the left making his group strong and black's weaker, white is now saving his top stones with attitude, instead of just making a useless heavy group.
$$Bcm6
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . 1 7 6 . 8 . . . . .
$$ | . O X . X O . . a . . .
$$ | . . O X X O . X . X . .
$$ | . O . O X . 2 . . . . .
$$ | . . O O . . . . 3 . . .
$$ | . . . . . . 4 . . . . .
$$ | . . . . . . . . 5 . . .
- Click Here To Show Diagram Code
[go]$$Bcm6
$$ -------------------------
$$ | . . . . . . . . . . . .
$$ | . . 1 7 6 . 8 . . . . .
$$ | . O X . X O . . a . . .
$$ | . . O X X O . X . X . .
$$ | . O . O X . 2 . . . . .
$$ | . . O O . . . . 3 . . .
$$ | . . . . . . 4 . . . . .
$$ | . . . . . . . . 5 . . .[/go]