Today's commemoration of Gregory of Nyssa brings a quote from his Great Catechism (ca 383).  It talks about the fact that Jesus is truly present in the consecrated bread and wine of the Eucharist, but in typical Orthodox fashion, it does not greatly elaborate the manner in which this happens.  The translation used is found in The Faith of the Early Fathers, vol 2, WA Jurgens ed, The Liturgical Press 1979, p 49

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Since it has been shown that it is not possible for our body to become immortal except it be made participant in incorruption through communion with the Immortal, it is necessary to consider how it is possible for that One Body, though distributed always to so many myriads of the faithful throughout the world, to be whole in its apportionment to each individual, while yet it remains whole in itself...This Body, by the indwelling of God the Word, has been made over to divine dignity.  Rightly then, do we believe that the bread consecrated by the word of God has been made over into the Body of God the Word.  For that Body was, as to its potency, bread; but it has been consecrated by the lodging there of the Word, who pitched His tent in the flesh.  From the same cause, therefore, by which the bread that was made over into that Body is made to change into divine strength, a similar result now takes place.  As in the former case, in which the grace of the Word made holy that body the substance of which is from bread, and in a certain manner is itself bread, so in this case too, the bread, as the Apostle says, "is consecrated by God's word and by prayer"; not through its being eaten does it advance to become the Body of the Word, but it is made over immediately into the Body by means of the word, just as was stated by the Word, "This is My Body!"...In the plan of His grace He spreads Himself to every believer by means of that Flesh, the substance of which is from wine and bread, blending Himself with the bodies of believers, so that by this union with the Immortal, man, too, may become a participant in incorruption.  These things He bestows through the power of the blessing which transforms the nature of the visible things to that of the Immortal.