Invasion as a Ko Threat
An invasion of your opponent’s moyo is a very large ko threat. If you can place a stone in your opponent’s territory that could live if unanswered, the value of the threat will be at least 17 points, and usually will be more.
However, the true value of the invasion ko threat might not be in the invasion living, but what it yields even when dead. Big dead groups usually contain several ko threats to live. Gaining four or five ko threats during a ko fight is not a small gain.
Here White should be able to kill the invading black stone, but the initial ko threat is worth about twenty points, and should therefore probably not be ignored.
While the invasion has been answered, it still has more ko threats to live, so the process repeats itself.
Here White plays to destroy eyespace while remaining somewhat connected. Black’s next ko threat is at a, and his invasion will provide several more threats before being unconditionally killed.
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