From an interview that included Nishimura Sensei
AikiNews Issue 87 -- Nishimura Sensei wrote:
It is necessary to relax your shoulders. You should not rely on power from the beginning. This is what I heard from Mr. Saburo Wakuta, Tenryu. He said that when Ueshiba Sensei was sitting on the mat, smiling, and he tried to push against his back, it was he who slid backwards.