Henry has a different perspective on this and it is based around the concept of Yin and Yang .
The black and the white sides represent the Uke and Nage. Each has a center, however during the contact a new center is realized. This new center is maintained by the Nage staying consistently extended through-out the technique, neither pulling in or over extending, but constantly maintaining the 50/50 balance of Yin and Yang.
To maintain this balance and the Center, when the Uke applies more than their 50% during the attack, the Nage cannot pull in because their 50% just went to 45% or less. They need to stay extended and maintain the center by moving their feet. Whether the move in the opposite direction or include an angle or a dip of their center, staying extended, and using their feet, keeps the 50/50 balance and the center between the two practioners.
