What is "Direct Penetration" from The Book of Five Rings book of Water?

Question

The translation of the book of Water of The Book of Five Rings at http://www.bookoffiverings.com/WaterBook.htm states:

The spirit of Direct Penetration is handed down in the true Way of the Ni To Ichi School. Teach your body strategy. You must practice well. This is transmitted orally.

It has been a few centuries since this was published - is this now recorded, has it been lost, or is there some unlikely school which still retains a legitimate oral tradition? If it has not been lost, what is the "spirit of Direct Penetration"?

Answer 1

The following translations (by Victor Harris and Thomas Cleary) instead translate the phrase as "Direct Communication":

The spirit of "Direct Communication" is how the true Way of the Ni To Ichi school is received and handed down.

The corresponding passage in Japanese (possibly modernized Japanese) is:

わたしは、勝利に直結するあり方を、わが流派の道としてお伝えします。

よく練習して、わが流派をきわめて下さい。

詳しくは口頭で伝えますね。

I don't speak Japanese, but Google Translate gives the following:

I will tell you how to directly lead to victory as the way of our school.
Practice often, please refrain from our school.
For details, I will tell you verbally.

Looking at translations of individual words, and the context, I'd say that this is not a reference to some secret technique, or school of fighting, but a simple statement that he's talking straight, not producing flowery metaphor from which someone is supposed to figure it out for themselves.