In an interview by Meik Skoss of Sawada Hanae, in Koryu Bujutsu, Classical Warrior Traditions of Japan. He asked her about aikido technique, she said: "When you speak of matching, if your kokyu, or breathing, does not match then you do not match. You are thinking merely of form, aren't you? But in order to do aiki, both your spirit and that of your partner must enter into play and then come together. When you study aiki, this is what you are studying."
Kokyu seems to be an integral part of Aiki(do) from this.
You know, I would be careful of quotes like this. I have mentioned this before a number of times, but the translations of a lot of westerners who don't really have kokyu/jin skills or understand exactly what is meant idiomatically by "ki" or words like "kokkoro" when they are used to indicate the intention-strength... those translations can be mis-leading.
In fact, pretty much the whole Chinese martial-arts world was shot in the foot for so long because of similar misunderstandings about "jin" and "energy", "qi" and "spirit", "breath", and so on. Not saying the translation is wrong, but I'd be very careful of it until I determined what the translator actually knew in terms of skills and so forth.
As we've witnessed in the last few years, there are a LOT of learned opinions by 'seniors' on simple ki/kokyu skills that have not panned out in terms of results/functional-ability. I.e., I'm saying be careful before you wander the graveyard of sayings looking for a headstone that agrees with your own perceptions.