the altar fasts when we fast: The Liturgy of the Pre-Sanctified Gifts

What is the Liturgy of the Pre-Sanctified Gifts in the Eastern Churches? Why do the Eastern Churches refrain from a Divine Liturgy (Mass) during the weekdays of Great Lent? 

"The Eastern Churches will not have a full Eucharistic service on major fasting days. There is no consecration on these days. Some will say - the altar fasts with us. During the Great Fast this applies to all weekdays. In order to have a communion service during the week, extra bread is consecrated during the Sunday Divine Liturgy. These pre-sanctified gifts are set aside for use during the week. The Liturgy of the Presanctified Gifts is therefore a fasting event. The tone of the chant changes to one which is more somber; the pacing slows down a bit; the readings are mostly Old Testament. In the Divine Liturgy, we are raised up to heaven and stand in the presence of the Divine. In the Liturgy of the Presanctified Gifts, we prostrate ourselves with Adam at the gates of Paradise, so close that we can taste what we are not worthy to receive." Father Brian Norrell