secret method; secret formula
General term for a confidential technique, recipe, or process, often in cooking, manufacturing, or magic.
This sauce is made using a secret method passed down for generations.
esoteric rituals (Shingon Buddhism)
Specifically refers to the secret, esoteric practices in Shingon Buddhism, not a general secret method.
He never revealed the secret method behind that magic trick.
Shingon Buddhist monks strictly observe the esoteric rituals.
Compound of 秘 (secret) and 法 (method, law). The Buddhist sense is a specialized use of the same characters.