The origin of the secret of the eighth inning

From the book The Secret of Special Doors. "Eight innings" does not mean eight innings, but eight important principles or formulas. The Secret of Eight innings is from Zhang Jinhai's book The Secret of Special Doors. At that time, a warlock named Zhang Jinhai was proficient in Daoism, and based on this, he wrote a book called "The Secret of the Special Door", which focused on "Eight innings" and elaborated many important Daoism principles and formulas.