If you are cooking vegetables with minimal liquid, it is essential to keep them tightly covered to prevent them drying out and to allow the steam to build up in the pan and cook the vegetables. Green vegetables such as shredded cabbage or green beans can be cooked very quickly by plunging them into plenty of boiling water and leaving them uncovered so that the volatile acids which escape while they are cooking will not condense on the lid and fall back into the pan, as this can affect the flavor. This method conserves their color, flavor and texture.
Roots and tubers such as potatoes, which are submerged in water and cooked for a longer time, are best partially covered. Covering these vegetables uses less fuel and prevents a lowering of the water level, which could leave some of them above the water. To partially cover a saucepan, set the lid askew or place a wooden spoon across the pan and balance the lid on top.
** Asian Recipes