I like seeing comments about visualization tricks during practice, even if I don't agree with the specifics of the visualizations in question. I just like the fact that it's seen as having a functional effect on the body, by controlling the directions of applied forces (rather than being fluffy nonsense).
Anyway - Szczepan, I like your comment about the 2 forces that make ikkyo. I personally think of this in more general terms, rather than your description of these 2 specific things. But for the specific case of ikkyo, my idea fairly closely matches yours. I was excited to read that.
But I'm surprised-- do you think the videos you posted show these 2 forces? I'd say the omote video very strongly demonstrates them, especially in the 2nd example at 13 seconds in. But, the ura video shows the "hip" one fizzling away.
Of course, in omote this "hip" area force vector strongly drives forward, and can end up displacing uke's position in a visually obvious way; whereas in ura it more subtly presses in. But, press in it must, even if not visually obvious.
It just seems to me that it stops pressing in, during the ura video.
What do you think?
Let's first put our discussion in correct context of this topic - a beginner is asking questions about very basic aspects of ikkyo. From my experience, any complicated explanation is useless, he will understand nothing at all. What is needed in such situation is a one general idea(lack of unbalancing) to give him a global direction for his future training and a simple physical solution that he can use immediately to unbalance attacker. That was my reasoning when I provided some advice.
Now, I used term 'vector of force' - it is not a 'trick' - it is a real geometrical line you can draw on the tatami relatively to the position of the line of attack and direction of the hips of attacker. You may call it visualization, I usually put the jo on the tatami to illustrate these lines, so students see it clearly.
These videos are used as a help to preparation 5th kyu test, to associate a movement with a name, and to get general idea what is difference between omote and ura., At this level I don't expect they will perfectly use these vectors.
Your questions are on more sophisticated level. Ura version looks easy but in reality is very difficult for me. The reason is - both vectors exist, but inside of the spiral that going down. So while nage turns his hips to create a spiral,he is pushing attacker arm outside using a lock, to 'catapult' his hips outside of this spiral.
It is true that nage hips are not pressing much attacker hips - ura version has, as a base, not to disturb too much attack, rather keep company to it. This is done to not display too early nage intentions.
Another foundation of ura is to create with a spiral kind of 'black hole' where weight of attacker will push his body. So the use of the vectors much take it in consideration.
Sorry for complicated answer.