Well first let me point out two things, eventually two very different things though in the beginning called and seen as the same thing. LEADING.
There is leading the mind and there is leading Ki.
O.k. So he is trying to show the effect of leading the mind and some principles involved in that aspect. Which brings me to the down/up effect.
I can see you know the principle of push down the reaction will be up so you translate it as such, action/reaction. All good physically.
So back to the demo. Leading the mind Tohei style is difficult and takes time to understand in itself let alone then use the principles of it. Add to this that there is another factor of weight underside, (which comes from complete relaxation and at the same time 'flexible' yet 'unbendable' arm) involved. (as shown in his ikkyo exercises)
Now here's the corker. Down/up mind wise. If you lead and then drop the persons mind they will pitch foreward. It's a drop straight down. verticle, no nearly verticle, so he is dropping their mind.
Now to phase two, I call it the tennis ball effect. If you have learned to lead the mind you then learn how to bounce the mind. The effect on the opponent is not one even from their perspective that they are reacting against something they just feel taken and bounced.
I hope this gives you an insight as to what's going on. Try it physically and you can get a pitch foreward or even a jerk reaction back but you cannot get that same bounce, it's a totally different skill.
This aspect of Tohei's aikido is fundamental to his particular 'way', others may have it in theirs but it is maybe not so apparent.
I spend a great deal of time with my students examining and practicing these principles, and you are right, they are not that easy to grasp - very easy to do when you have embodied the skill, but difficult to get rid of all the mind/body habits that get in the way of it happening effortlessly.
For me, leading ukes mind from the moment that they initiate the attack, is what makes the whole thing work. Lead the mind and the body will follow. The tennis ball effect is exactly as you describe, both powerful and slightly disorientating for uke.
I really enjoyed watching the video as I had not seen that one before. I wish I could understand Japanese, to hear his detailed instruction, I certainly recognised all of the principles being shown.