Just an observation, but if you throw a left hand back fist to the left side of your opponent's head, your opponent can block with his own left hand and your left side will be open to a strike. Your right hand may be up but it won't be covering the left side of your head.
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If the left hand is the back hand then the right hand can cover the left hand side of your head. It would be the right hand side of your head that wouldn't be covered so you would bring your arm up so the elbow protects. Though we would tend to move as Wado is more of a 'body movement to get out of the way' sort of style. That's my words lol not the official description of Wado Ryu.