In English, we have an expression: "the secret sauce" to refer to a trick/hack/essential ingredient to doing something well. Literally, it is easy to see where this comes from – a food can taste very good because it uses a tasty (but little-known sauce). This expression can be used more generally too however (e.g. the "secret sauce" in a formula for financial independence/language study/etc.)

What similar expressions exist in Chinese to express this idea? I am aware of
秘诀 *mìjué*, but am wondering if there are more interesting/idiomatic ways to say this.