Silence is no passive thing. It is a vibrant presence which fills any vacuum in sound and conveys its own living quality. Most of us know the truth of the phrase "the silence that sounds," and the old adage "silence is golden" means more than just that it is safe or wise. It means that it is golden in the sense of being filled with light.
Fasting from words, fasting from a volume of speech, being willing to humbly accept the yoke of silence and quietude deprives the tongue of the mastery of our hearts and minds. The tongue is a very useful tool for the art of love and for the art of prayer, but it is also the means by which we afflict others and even our own selves so often. So it must be called into holy obedience.