This is one of the most important techniques in Chinese cooking. The trick is to regulate the heat so that the surface of the food is sealed but does not brown so fast that the food is uncooked inside. Although deep-fried food must not be greasy, the process does require a lot of oil.
The Chinese use a wok for deep-frying, which requires rather less oil than a deep-fat fryer, but it may be best to avoid using a wok to deep-fry unless you are very sure of it. If you do choose to use your wok, make certain that it is fully secure on its stand before adding the oil and on no account leave the wok unsupervised.