The secret ingredient in stir fry beef is a Chinese technique which is known as velveting the meat, that uses a mix of oil and cornstarch and sometimes even some egg whites or baking soda.
To make great stir fry beef, you should use the Chinese technique of using a mix of the oil and cornstarch and even baking soda or egg whites, which tenderizes the beef and creates a protective layer, which keeps the beef moist and silky when it's cooked quickly in a hot wok.
The key when making great stir fry beef is the coating and tenderizing process before cooking the stir fry beef and not just a single ingredient in the sauce that is used.
The baking soda is what breaks down the meat fibers in the beef for extreme tenderness, although to avoid a soapy taste you should use it sparingly and use only around 1/2 tsp per lb.
Egg whites in the stir fry beef seal in moisture and creates a nice silky texture that is often used with cornstarch.
The cornstarch for the stir fry beef forms a protective barrier, which locks in moisture and gives it a smooth and velvety finish.
And the oil is added to the stir fry beef marinade to help with the process of velveting and prevent sticking.
Soy sauce, garlic, ginger and oyster sauce is also essential for flavor of the stir fry beef, but velveting the beef first is the truly secret technique that restaurants use for making exceptional and tender stir fry beef.