In the first picture, from another demonstration you can most clearly see the arm that is keeping "CXW" from being pushed over. It can again bee seen in the strongman, however the camera guy did a good job with the heavy guy and we really only catch a brief moment of the arm (I had a hard time getting it) In the first video I found of CXW doing this the arm that helps is shown often, but in this strongman video you can't see it very often, it's alway in the back.
I found a few video's of CXW doing this kind of push test, in every example he has his arm in the same place. Is it possible that this is not how he's doing it, absolutely. However if you put your arm in that spot, you can do exactly what he is doing, maybe not quite as well (without some practice), but you can do it too.
While I've heard people sat that this can be done with the arms down I haven't seen any video. If there is video of such a thing I would like to discuss that video.
His arm is not where you show in your diagram. I learned that little trick forty years ago. It's a little trick. It's NOT what's being done in the IP/IS training.
In these pics, CXW's arm is over the other guy's arm--not
pushing up from underneath.