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Bisbigliando ("whispering" in Italian) is a special tremolo effect on the harp where a chord or note is rapidly repeated at a low volume. Usually, the effect is achieved by both hands playing adjacent strings set to the same pitches. This is possible because of enharmonic spellings created by the pedals. It can sound like strumming a guitar or even "watery".
On sheet music, it is represented by three thick lines connecting the notes to be trilled.