Bisbigliando is a tremolo between different fingerings of the same pitch, often referred to as a timbral (color) trill. The result is a fast, shimmery change of tone color. No Bisbigliando is possible in the lowest octave of the instrument. Some pitches have a larger variety of different Bisbigliandos.
As Bisbigliando is based on the Harmonics of the flute, it is a tremolo between different fingerings of the same harmonic pitch. There are dictionaries of fingerings for Bisbigliando, and you can experiment with your own flute to find fingerings that work for you.
Bisbigliando in succession (see musical example)