Tapping, Open Strings

With the electric guitar, in this situation the output tone itself is usually overdriven although it is possible to tap acoustically with drive serving as a boost to further amplify the non-picked (and thus naturally weaker) legato notes being played. Because of the amount of distortion generally present, the player should also focus on reducing unnecessary noise during tapping; for instance, by using the palm of the tapping hand to mute any open strings that might otherwise ring out.

Source: Wikipedia > Tapping