Really? So if he stands in front of me, feet apart (shoulder width) and I push him backwards he can withstand that force without stepping backwards, without losing posture? Seriously? SHOW ME!
Seriously....Iv'e felt it. Iv'e done it. I can't do it consistently ,yet, but it has started to be integrated into my aikido, especially in ukemi. Enough so that a 6th dan commented to my teacher at a seminar this weekend that I am very hard to move.
In particular it comes in really handy when you need to freeze in a very awkward and unbalanced position while sensei explains something. Those opposing forces work wonderfully for keeping me from collapsing in a heap prematurely.