To be a contemplative is to be a watch in the night for the approach of Mystery. And it is a readiness to be transformed in an engagement with that Mystery.
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.