Two things: That's what I've been told by Toby Threadgill - AND - Akiyama as well as the founder of Yoshin Koryu, in particular, traveled to Southern China.
Parenthetically, the texts of Chinese military tactics that were disseminated in Japan all antedate any of the texts extant on such arts as t'ai chi, bagua or xingyi. They are, to my understanding, all Shaolin based.
I do not know if Scholar Boxer: Chang Naizhou's Theory Of Internal Martial Arts And The Evolution Of Taijiquan
- the translation and commentary by Marnix Wells, was disseminated in Japan at all, and this book was written in the 1700's. Marnix's translation is extremely awkward, sad to say (yin and yang are translated as shady and sunny), and the art that is described is very rare, and one has to imagine what is possibly being talked about. At any rate, this is the first text, as far as I know, that is considered to be about something approaching purely internal training.
Specifics beyond that would require an examination of the training exercises of Yoshin-ryu and a comparison with those of various systems of China. And given the traditional teaching methods which Yoshin-ryu maintains, given the more essential teachings to initiates only, good luck