Mel Barker wrote:
My addition - nage should squat to raise his arms rather than use muscular strength. Using the legs to raise uke's katana (arm) is most effective and powerful.
That's a whole other can of worms. I would never advocate doing any technique with just arm/shoulder girdle strength, but to do this or any technique where your arms are not as limp as wet noodles requires the use of muscular strength.
I think you are framing it in terms of a false dichotomy. It's not use center/legs OR use upper body muscular strength, but use upper body muscular strength properly coordinated with center/leg movement OR use upper body muscular strength improperly/uncoordinated with center/leg movement.
If you were to analyze the movement in excruciating detail with high speed video, EMG sensors, etc... you'd see that what you're talking about is more about using the upper body/torso muscles isometrically to transmit dynamic force/movement generated by the lower body as opposed to using them to generate the dynamics independent of the lower body.
...might seem like tedious nit-picking, but I think broadly inaccurate hyperbole like this and oft-repeated 'relax' mantra can be really confusing to the relentlessly skeptical and literal-minded. I know it was for me.