Epiklesis

Definition

The [Greek] word means [invocation] and refers to the [prayer] that originally [appear]ed in all [rite]s of the early [church] and continues in the [Orthodox] [church]es, but it has been dropped from most of the [liturgy|liturgies] of the west. It's place in the [Latin] [Mass] was after the words of [Institution] that begin Hoc est enim corpus meum..., ("This is my Body..."). In the Epiklesis the [celebrant] [pray]s that [God] will send his [Holy Spirit] to [change] the [bread] and [wine] into the [Body] and [Blood] of Christ. An example of the epiklesis is taken from [Orthodox] [Liturgy of St. John Chrysostom] that reads in part:
[Priest] (silently): Again we offer to You this [spiritual] and unbloody [sacrifice], and we implore and [pray], and [entreat] You, send down Your Holy Spirit upon us and upon these [gift]s here present. (Blessing the bread) And make this [bread] the precious body of Your [Christ]. (Blessing the chalice) And that which is in this chalice, the precious blood of your Christ. (Blessing both) Having changed them by Your Holy Spirit:

Controversy

And here lies a great controversy between the Eastern and Western churches. If the "magic" of [transubstantiation] takes place at the words of Institution as maintained by the Roman Church, then the Epiklesis is unnecessary. If, however, the transformation of the elements of bread and wine into the Body and Blood of Christ takes place during the Epiklesis, then any rite without it is defective in an essential part. As I understand Orthodox theology, what happens at the Epiklesis is not as finely defined as transubstantiation in the Roman Church. Still, there does not seem to be a [resolution] to the [problem] of what happens when.
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References:
http://www.newadvent.org/cathen/05502a.htm
http://www.byzantines.net/SaintAthanasius/liturgy.htm

Epiklesis (idea)