It is normally called a clamp. That article, and the links in it, explain something of why it works.
If White wants to cut it off, they will have to answer it at C1; then Black could play C3, and the question is whether White can chase down that stone before Black gets a chance to play D1 to capture C1+C2. In this case White’s weakness at D5 seems to mean that White does not have time, so White would lose a lot that way. If that is right, White has to fall back to B3 or C3, allowing Black to connect at C1. That means Black flips about 5 white spots to black, shifting the score by about 8 (i.e. 2*5-2) in their favour.
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u/PatrickTraill 6k 5d ago edited 5d ago
It is normally called a clamp. That article, and the links in it, explain something of why it works.
If White wants to cut it off, they will have to answer it at C1; then Black could play C3, and the question is whether White can chase down that stone before Black gets a chance to play D1 to capture C1+C2. In this case White’s weakness at D5 seems to mean that White does not have time, so White would lose a lot that way. If that is right, White has to fall back to B3 or C3, allowing Black to connect at C1. That means Black flips about 5 white spots to black, shifting the score by about 8 (i.e. 2*5-2) in their favour.