"Bamboo joint" is a term from the game of Go (Wei Qi/Badouk). It refers to the shape formed by the stones as shown below:
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This shape can be good or bad, depending on context. It's a very strong shape, but also overconcentrated. It doesn't form any eyes as it stands, but has the advantage that it can't be cut. The two empty points inside are miai for connection; if the opponent plays on one, you can respond on the other and stay connected. This gives rise to the proverb "Never try to cut a bamboo joint." It also makes it a good shape to play during a battle where strength and connection are important. It is not efficient to play this shape if there are not many other stones in the vicinity.