In this space, this void, there exists everything and nothing. Both peace and conflict can arise. And anything in between.
If you don't find this idea in your training then you miss out on a great power in our art and in life.
Hatsumi Sensei gives us a roadmap to understand this in the following quote:
It is taught that the foundation of Budo is to first understand taijutsu, through which you can fight even if you have no weapons. This means to persevere in the martial ways (bufu ikkan), and to train consistently and with utmost effort. Then you will grasp the secrets of muto dori (no-sword technique). Succeeding in this, the mysteries of the secret sword (hiken) will be revealed, and no matter what weapon you hold, your heart and your taijutsu will dance skillfully in the void (koku).
While these ideas may seem esoteric or advanced, they have great practical application. If you dismiss them as something apart from the reality of training or combat then you miss a very large variable that can harm or protect you. The void doesn't take sides.
Paul Masse reminds me of this idea in his blog:
(妙術、実）あると思えば,ない. ないと思えば, ある。If you think there is something, there is really nothing. And when you think there is nothing, there is something.
Ignoring ideas about Ku in training can be the same as ignoring how to punch or kick. You need both. And nothing.
Soke continues to say:
Facing an opponent, armed with a sword, adapting to change (henka), hiding in the void (koku), accepting change, and acquiescing to the void - this is never about killing the opponent or benefiting from the aggression of your allies.