from https://www.rojodojo.com/be-an-insect-of-victory-or-kachimushi/
Takamatsu Sensei used to say that even a small fly could go go to the ends of the earth if it held onto a horses tail.

That's what we are doing if we follow Hatsumi Sensei's teaching. We are trying to hang on to a galloping horse. But the horse is not Soke. It is the many generations of tradition and the spirit of Ninpo that is greater than any of us.

Even Hatsumi Sensei himself has stated that his fifty plus years of training doesn't mean much. He feels he is still walking along behind Takamatsu.

Soke also reminds us that a horse's tail is actually used to flick away pests. So if you don't respect the nature of what you have grabbed onto by training in the Bujinkan, you may be in for an unpleasant journey or swatted away.

Once you have a feel for the nature of the ride you are on, the trick is learning how to hang on. It can't be done with strength or willpower.

Maybe this is one meaning of Bufu Ikkan.