Having practiced both Iwama and Aikikai, I can say that the Rokas guy from AikidoSiauliai/AikidoJourney is definitely not Iwama (nor any of the older styles of aikido). He looks like a shodan (1st degree BB) from a modern school of aikido.
This is not a definition of aiki. This is a definition of the "ju no ri" of judo (pull when pushed, push when pulled), at least in my understanding of ju no ri. Aikido can also make use of this concept (particularly in dynamic situations) but it is not aiki. I would like to add that given the number of high-level judoka (and other martial artists) that were thrashed by Osensei and described his strength as "a mysterious force that is impossible to resist against", I doubt that they would have had this impression from an application of the most basic judo principle (the same discourse can be applied to Yukiyoshi Sagawa and Sokaku Takeda).
While in a real situation it will be applied upon contact (creating "instant kuzushi"), aiki at its most basic levels is trained in static situations and does not rely on timing. Here is the kokyu ho exercise, which has been called "the secret to aiki" by aiki's best exponents and can be trained from both static and dynamic POVs:
Based on quotes from Morihei Ueshiba, Yukiyoshi Sagawa and some contemporary sources (that demonstrate, for example, that Osensei was using the same terminology and imagery as the ones used in the Tai Chi Classics), my current understanding of the concept of "aiki" would be as follows (I will expose here the different aspects/levels that I can think of, even though they ultimately overlap):
- the ability to manifest a tension between opposing forces within the body (extension-contraction, gravity-ground reaction force, etc.)
- the ability to absorb external force into the center, make it rebound into the ground and send it back through physical contact, nullifying the force of an opponent thanks to "non-resistance"
- the ability to redirect the nullified force by tackling it at a perpendicular angle (we use circular or spiral movements so that the force applied to the point of contact is in a direction that is tangent to the circle, thus perpendicular to the force of the opponent)
- the ability to focus whole-body power into one movement (this, in conjunction with the use of momentum, forms what is called "kokyu-ryoku")
I'm purposefully leaving out the spiritual/tactical aspects (being one with the universe, "filling" the voids that the opponent makes within his own guard when he moves, etc.) because, as you see, it is a really convoluted, complex, and holistic principle and the best one can hope for is to make artificial distinctions such as the ones above.
Some styles/teachers focus more on some aspects (Ki Society focuses a lot on push tests and what can be linked to the two first aspects, while the kokyu bit is emphasised in Iwama and Yoshinkan, etc.).
Based on videos, Osensei seemed to focus on a more dynamic application of aiki compared to the Daito ryu guys (what Iwama guys call "ki no nagare") and that might be the thing that led to the confusion that aiki is "good timing" or that aiki is "ju no ri" (or "blending"...). But he was on a different level also in terms of perception and reflexes. Leaving out the stories where he reportedly dodged bullets, he was able to catch the punch of a pro-boxer (Piston Horiguchi) and pull him to the ground with the first technique in this video:
There's so much more to aiki that frankly I can't wrap my mind around it.
For a more contemporary example of good technique, relaxation, non-resistance, connection and of how the power can be transmitted from the hips to the hands via a relaxed structure, I really like Ryuji Sawa's aikido (he's from the Iwama school where, although they don't do full-on sparring, uke is told to resist fully to the technique):