The saying goes: "There is ...
By Rob Van Zeijst
The saying goes: "There is no such thing as a free lunch." In go, there is a free lunch, but you must eat it first. In other words, there are many instances in which a player can profit by moving first, as his opponent is forced to answer.
Double sente
Many moves are sente (keeping the initiative), and even double sente, i.e. either party can play a specific move to keep the initiative. Therefore, if it is your move, why not play it immediately? Well, it is all a matter of timing. It may be sente, but other moves may be more important.
Diagram 1: In this situation, black 1 seems to have the initiative. White will almost certainly answer to prevent Black from playing A for B in sente. After all, this could impair White's eye-shape in this area. After the 1-2 exchange, we can assume Black will play at C. However, there is a possibility White will play at C first. Black may or may not answer at D right away. If he does, White probably will lose his stone at C.
Diagram 2: In this case, white 1 is sente. Black usually will answer with 2 to avoid the sequence white A, black B, white C, black D, white 2 and black E. Later on, we can assume white will play at F. But we have to consider the possibility of Black playing F while White plays at G.
The difference between Diagram 1 and 2 is two points, but this will depend a little on white C in Diagram 1 and black F in Diagram 2. However, the point is that these moves are sente for both sides, so whoever plays first profits. But if there are more significant moves on the board, these moves will be sente for neither player.
Diagram 3: What about this situation? Black's three marked stones are alive. However, since this group is surrounded, Black will have to defend if threatened.
Diagram 4: White can threaten, starting with 1 and 3, and Black has no choice but to defend all the way to 12, or else his group dies. Black has lost a lot of ground--his corner territory is only about 12 points.
Diagram 5: With the marked stones in the same place, the situation is quite different. Black is strong and it is White who has problems. Black gets all the sente moves 1 through 8. Black's corner territory is about 18 points, while White's territory has shrunk. In this diagram and the previous one, the player who initiates the exchange keeps sente. So, that player gets something for nothing--a free lunch.
Diagram 6: Going back to our first situation, White could also strike at 1. This looks like a strange move as it can be cut off. But note what happens. After the sequence through 9, White is alive and Black may die.
Diagram 7: After white 1, Black caves in with 2 and 4, and the sequence 5 through 10 is sente. If we assume the sequence white A, black B, white C, black D, white E, black F, we can determine that this is better than Diagram 4 by about two points.
Diagram 8: White can also start at 1. Even if Black fights back with 2, White is alive in the sequence through 9 and Black is in trouble.
Diagram 9: Again, Black has to give in with 2 and 4, and loses territory all the way to 10. Later on, White may get to play A, followed by black B, white C and black D. That reduces Black's territory even more while enlarging White's. This sequence might be best, but it all depends on the circumstances.
Conclusion: It sounds obvious, but sente moves depend on the opponent answering these moves, i.e. when nothing more important is around. At the same time, if the timing is right or a move is important, sente moves will be answered. There is also another problem with playing sente moves too early. They erase the potential for other possibilities.
New Problems
Problem 1: Where is a big double sente move?
Problem 2: How can White threaten Black's corner and profit? There are several possibilities, so after you find one, look for others.
Problem 3: We talked about these three marked stones before. Instead of defending at A, there is another possibility. What is it?
