The strong player plays straight, the weak plays diagonal
Dieter: It seems that this proverb doesn't have its own place yet. In Haengma tutorial for beginners, Minue makes a strong point about this, however without giving the fundamental reason why straight lines are better than diagonal ones.
Anyone?
Velobici: Diagonals have cutting points. Cutting points make the position more complicated and plant the seeds of future reading errors. Diagonals seem to add two liberties (labeled a in the diagram), but threatening to cut removes two liberties. Black can restore the prior number of liberties (labeled b) by playing
which makes a straight shape from the marked black stone.
By contrast, playing ,
,
is a turn; straight shape from
to
. A turn is often powerful and show why pushing from behind can be so bad. White has induced Black to play the turn. If cut, each group must live independently or kill the cutting stones. Assuming the attack is launched from/to strength, then one of the attackers two groups is already strong, while both of the defenders are weak.
Straight shape does not have cutting points and adds two liberties per move (labeled c). Yang Yilun emphasizes the usefulness of straight shape in defense.
Diagonals (hane and keima) are good for attacking. Straight shape is good for defense. To attack successfully, one must start from an advantageous position. For each move played the weaker party can gain strength faster for the weaker party must start with fewer stones, or fewer useful, effective stones. Straight shape helps to create such positions (wall).