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Chapter 8 of Riichi Book 1 (available for free here) does a great job of explaining the fundamentals of defence. Then to directly practice you can use Erzzy's folding trainer and it corresponds directly to the levels of safety described in riichi book 1 (this is a link to her efficiency trainer, but a folding trainer is available within the same thing once you run it). For people that don't speak Japanese, riichi book 1 is the go to answer for learning any of the fundamentals of the game :) .
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