The term wok hei (Cantonese: 鑊氣) is both a practical description of an identifiable aromatic quality and a shorthand for the entire skill set of Chinese wok cookery at high heat.
Wok hei — literally 'breath of the wok' — is the complex, smoky, slightly charred aromatic quality imparted to food cooked in a screaming-hot seasoned wok over a ferocious flame. It is the single most discussed characteristic of Chinese restaurant cooking, and the principal reason home stir-fries rarely achieve the depth of a professional kitchen. Compounds responsible include Maillard reaction products, pyrolysis of residual fats on the seasoned wok surface, and partial combustion of volatile aromatic compounds at temperatures exceeding 300C. Grace Young in The Breath of a Wok devoted an entire book to its science.
Wok hei is not a single flavor but a family of aromatic compounds that together produce the sensation of freshness and vitality distinguishing restaurant Chinese from home Chinese.
Three non-negotiable conditions: (1) A very high flame — professional wok burners produce 65,000-130,000 BTU vs home burners at 8,000-15,000 BTU. (2) A carbon steel wok with established patina. (3) Minimal food in the wok at any given time — a crowded wok steams rather than sears. Pre-heat the wok until a drop of water evaporates within 1 second. Add oil, swirl to coat. Add ingredients in sequence based on cooking time. Toss rapidly using pushing-and-tossing motion to expose all surfaces to the wok's hottest zone. Allow brief contact pauses — 10-15 seconds without movement — to build the Maillard crust. Home approximation: Use the smallest wok for the food amount. Work in small batches. Never add cold liquids. Dry all ingredients before wok contact.
Carbon steel woks are preferred over cast iron for their lighter weight and rapid heat response. A wok ring stabilizes the wok on Western stoves but kills heat transfer — use a flat-bottomed wok on gas without a ring. The seasoned patina is the accumulated flavor of thousands of meals. Never wash with soap.
Overcrowding the wok: The single most common error. Even one portion too many causes steaming rather than searing. Insufficient pre-heating: The wok must be visibly smoking before oil is added. Adding wet food: Wet ingredients immediately drop the wok temperature by 100C or more.
Grace Young, The Breath of a Wok (2004); Kenji Lopez-Alt, The Food Lab (2015); Fuchsia Dunlop, Every Grain of Rice (2012)