Find the Opponent's Weakest Spot, and Hit it Hard
Black to play in Dia 1
Two things that should be obvious at at first glance of this position are the huge black wall in the upper right (whose influence is somehow defied by the lonely white stone in the 7x7 location in the upper left)), and the big white potential in the lower right side.
Is there a way for black to attack the lonely invader?
Also, White's shape in the middle of the right side looks "unfinished": maybe black can do some damage there, as well.
Dia 1
Black can't actually attack the white invader in the upper left too strongly: the reason is that black himself has several weaknesses in that area.
What black can do, though, is taking advantage of the fact that white missed to defend at 5 (or 6) in Dia 2
Dia 2
Black has cut through White's main moyo - in sente! White needs to defend at 10, otherwise his upper right group is missing eye shape.
Black 13 is just an example - black can choose to play somewhere else. The main point is the sequence up to 12, where black reduces about 15 points off White's side territory in sente - which is equivalent to a 15 x 2 = 30 points move in gote. This way it is "game over" for white.
At least this is the professional's view: Tei Meiko 9 dan commented on this game of mine and explained all these. I would lie to say that I really internalized this lesson myself and really understand that it's game over for white - I guess this is just one more thing that makes me not be a 9 dan pro :-)
Dia 3
Conclusion
- One should be quick to take advantage of opponent's weaknesses - even one move too late may be an opportunity missed forever.
- It is very good to cut through your opponent's main territory - and twice as good if you do it in sente.