Did you never ask any of your teachers? Not that they would probably have told you (assuming they could have, not a given).. Just curious.
Hope you are well. Funny I never got around to asking Tamura Sensei how he did what he did.As for asking my other teachers I have as yet not asked them.Some of those who might have been able to give me an answer have passed away. Sekiya Sensei for example held me down with a kesa gatame [scarf hold in Judo ] and I felt he weighed a ton.Barely budged him and he was not a heavy man.My own view is that the acquisition f what the Chinese call 'Heavy hands ' requires a total relaxation of the body and a certain mindset.If i do get a chance I will try to get some info on this subject.Unfortunately I may not be able at least for now to ask about this.Cheers, Joe