Guo you (過油, literally passing through oil) is the professional Chinese kitchen technique of pre-cooking protein or vegetables in a large quantity of oil at low-to-moderate temperature (110-150C) before the final stir-fry. The technique seals the surface of the ingredient rapidly, locks in moisture, sets the velveting coating, and allows the final wok cooking time to be reduced to seconds — enabling the characteristic speed and precision of Chinese restaurant cooking. Without guo you, a busy restaurant kitchen could not maintain quality across dozens of simultaneous orders.
The guo you setup: Oil volume sufficient to submerge the ingredient — typically 500ml-1L in a wok. Temperature: 110-150C for most proteins. Vegetables pass through briefly at 160C. Time: 30-90 seconds for velveted chicken or beef. What guo you achieves: (1) Sets the velveting coating into a smooth silky layer. (2) Partially cooks the protein — typically to 70-80% done. (3) Removes excess moisture from the surface. (4) Allows the final wok dish to be assembled in 60-90 seconds. Home alternative (qiao shui): Water blanching at 80C achieves 80% of the guo you result without the large oil volume.
Temperature too high: Guo you at 180C cooks the outside and leaves the inside raw. Returning cold guo you protein to the wok: The protein drops the wok temperature and steams rather than sears.
Irene Kuo, The Key to Chinese Cooking (1977); Ken Hom, Complete Chinese Cookbook (2011)