JimG:
(The Eastern Churches believe that it is actually the epiclesis that effects the transubstantiation, whereas the Western churches believe that it is the Consecration which effects the change.)
JimG,
Great answers but a bit off on this last one…
Actually in the East we perfer to say that it is a Mystery but that it is the whole Eucharistic Prayer that effects transubstantiation (a Western word and idea that is not used in Eastern Theology) but when pressed we would say that the time of this change is at the Epiclesis as it is after the words of Institution and, as you said, actually calls upon the Holy Spirit to, not only, bless the offerings but to change them.
Here is the actual Epiclesis from the Divine Liturgy of St John Chrysostom, this follows directly after the Words of Institution.
PRIEST: Remembering, therefore, this precept of salvation and everything that was done for our sake: the cross, the tomb, the resurrection on the third day, the ascension into heaven, the enthronement at the right hand, the second and glorious coming again. We offer You your own, from what is your own, in all and for the sake of all.
PEOPLE: We praise You. We bless You. We give thanks to You, O our God.
PRIEST: Moreover, we offer You this spiritual and unbloodly worship, and we ask, pray and entreat: sand down your Holy Spirit upon us and upon these gifts offered.
DEACON: Bless the holy bread, Master.
PRIEST: +And make this bread the precious Body of your Christ.
DEACON: Amen. Bless the holy chalice, Master.
PRIEST: +And that which is in this chalice the precious Blood of your Christ.
DEACON: Amen. Bless them both, Master.
PRIEST: +Changing them + by your Holy Spirit.
DEACON: Amen. Amen. Amen.