Click Here To Show Diagram Code

[go]$$Bc
$$ -----------------
$$ | . . . . . . . .
$$ | . . X . . . . .
$$ | . . Q X . X . X
$$ | . . a b . . . .
$$ | . . . O . O . O
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . . X . . . .
$$ | . . . . X . . .
$$ | . . O , . . . .
$$ | . . . . . . . .[/go]

Click Here To Show Diagram Code

[go]$$Bc
$$ -----------------
$$ | . . . . . . . .
$$ | . . X . . . . .
$$ | . . Q X . X .
$$ | . . a b . . X .
$$ | . . . O . O O O
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . . X . . . .
$$ | . . . . X . . .
$$ | . . O , . . . .
$$ | . . . . . . . .[/go]

1) What is the right moment to defend the marked stone in that kind of local positions?
2) In what outside situations 'b' can be better then 'a'?

When it's the biggest move locally (and globally) ? Sorry, either this is a completely useless answer, or I don't really understand your question (1).

For (2), it probably depends on the whole board. As for (a) and (b), first feelings give these variations:

I think, in addition to second diagram, it might be good to consider this move as alternative later in game:

Click Here To Show Diagram Code

[go]$$Bc
$$ -----------------
$$ | . . . . . . . .
$$ | . W X . . . . .
$$ | . . O X . X . X
$$ | . . C . . . . .
$$ | . . . O . O . O
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . . X . . . .
$$ | . . . . X . . .
$$ | . . O , . . . .
$$ | . . . . . . . .[/go]

But you can choose to do this when you don't care about getting cut off with a black move at the marked intersection. Since there is some flexibility here (i.e. play for corner territory or just save stone normally), you should wait until you decide which one you like better.

_________________
be immersed

Another consideration is b can be a good way to resist the e14 peep.

Thanks for the replies. However, I still don't understand neither the difference between 'a' and 'b' nor the correct moment to play them. So let me put my question in another way. Please look at these professional games.

Here the questions are:
1) Why white 44 was at Q17 and not at Q16?
2) Why not play at Q17 earlier or later in the game?

Here the questions are:
1) Why white 30 was at Q4 and not at R4?
2) Why not play at Q4 earlier or later in the game?

Is it possible that the choice in these examples was just a matter of taste/style/mood?

In the first game, Black is strong all around and so White wants to settle his group cleanly. M13 prompted him to settle. In the second game, White wants sente, and Black isn't very strong nearby. He wanted to make a position on the right before returning to the bottom corner.