I well remember being told about the suburi, that each one is one. Neither the one before nor the one after should be in your mind, only the one you are making at that time.
It is easy to think you are doing just that with a ken strike, but try archery and realise just how difficult that is. Each arrow shot is one ... can’t be more simple than that, one arrow, one shot.
Where it lands in relation to where you hoped it would, or in the same place as the previous ..... or worse still, farther away. Watching where the arrow falls creates conflicts in the mind. Bad shot, we say idiot! Good shot ... we worry that we can’t repeat it. 36 arrows to shoot in a sequence of six by six ... each one to be consciously perfectly sent ... but when the shot is taken nothing other than that must be on your mind.
It is without doubt a great teacher for the ‘each one is one’ principle and far more obvious than the swinging of the ken while wondering what is for dinner.