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[edit] Tenkan: 転換
Turning movement, esp. turning the body 180 degrees.
This is achieved by sliding the front foot forward, then using the hips to turn around and bring the back foot all the way around, then the front foot closes to give a stable stance with 60% of the weight forward. Neither foot should leave the ground, and there should be a 'down' feeling during the entire motion, almost like a screw turning into the board. The upper body should stay straight, and stable, keeping good posture.
For attacks like tsuki, yokumen uchi, or shomen uchi, hands should be relaxed at shoulder level, with elbows down to protect side of body during the tenkan. For attacks like katatetori, or morotetori, curl the wrist to about a fists width from one point keeping the arm round, and keep it centered in front of one point during the tenkan.