Tai No Henko
"Tai no henko is the basis of all ura waza movement. The Founder said, 'practicing this tai no henko, you will build up strong and stable hips which will never be affected whether your partner pushes or pulls you'. If you do not have strong hips you cannot respond to powerful attacks. Therefore, build up such strong and sturdy hips that you will be able to move freely no matter how strongly your partner attacks you."
From "Budo: Commentary on the 1938 training Manual of Morihei Ueshiba"