Sanchin breathing is a really quick inhale with a "blow through the straw" exhale. It aides in keeping your body really really tight while making sure you're not holding your breath. Our Sensei told me that although it isn't necessary to do the "hiss" on the exhale they teach it that way to make sure the students understand the exhale portion of the breathing.

Quick inhale! No,wrong. Long deep breath in the nose. Long breath out with the arm extension,all while staying tense.
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