secret sauce

1 :  a sauce that adds an important element to a dish and that has ingredients which are typically kept secret A frank of beef, pork, and veal … is split lengthwise, coated with secret sauce, and cooked on a grill. — Jane and Michael Stern —called also special sauce

2 :  an element, quality, ability, or practice that makes something or someone successful or distinctive If leadership has a secret sauce, it may well be humility. A humble boss understands that there are things he doesn't know. — The Economist —called also special sauce