The trick to this is the timing of when to splay the fingers and extend into ones partner. There is also the aspect of relaxing and coordinating the whole body so that the rebound force is not lost or does not dissipate somewhere in ones own muscular structure.The force that is extended can be amplified within the body coming from the legs, waist and tanden through the spine and into the arms.
I'm interested - IIRC when Nariyama has been asked about kokyu his response has been "its all timing". I've often wondered where this perspective came from, and my own thoughts were pretty much along the same lines as you describe up there.
So as to your point about the "timing" - is that something that Shishida / whoever has explained and you're passing it along, or is it a discovery you've made by yourself and explaining as such?