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"?