In case someone doesn't read Japanese:
Oh and I have spent time with Yamaguchi S and been uke on many occasions. I once asked him if he knew what he was doing and he said no, not entirely. I also asked Tada S whether he thought Ueshiba had 内力. He answered that he thought he did, but did not teach it.
内力 is "internal power" (although I might also say 内勁).