secret sauce

[ see-krit saws ]
/ ˈsi krɪt ˈsɔs /

noun

a secret element, strategy, procedure, etc., that accounts for or increases the chances of success: The app's secret sauce is its simple interface.

Origin of secret sauce

from a so-called sauce for hamburgers whose ingredients are a closely held secret
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019