Hi there i am a member of sensei yahara's organisation KWF, and have trained with him many times. i have visited and trained with him in his dojo in japan for the last three years and believe to have a good understanding of his hip idea. in KWF karate, there are only two hip posistions, hanmi and gyaku hanmi. when making hanmi one must turn their hips as far as humanly posible. softening the back leg allows more 'give'. when, for example, making oi-zuki one does not simply keep the hips square, they must push as far as posible into gyaku hanmi. https://www.kwf.jp video archive dojo training will show you exactly what he is after. demonstrated by shihan yahara himself.