We had been talking about using three fingers for the Sign of the Cross about a week ago. It's something a lot of people either don't know or have simply forgotten over time.
However, after talking about it, he's more aware. I told him to look out for the priest at the end of Mass to bless with his three fingers raised because the priest acknowledges that the blessing is actually coming from the Trinity (God) and he is simply an instrument of His Blessing.
My friend then said, "Well why do they use their whole hand when they bless the bread and wine at Mass?"
I said, "That's a great question! They use their whole hand because they aren't really blessing the bread and wine; Christ is."
Since the priest acts in persona Christi, we understand (as faithful Catholics) that the priest, though present, is simply a vessel through which Jesus, Himself, comes to Consecrate.
The priest calls Christ from Heaven to consecrate. Only God can make God. Thus, the priest, being endowed with the privilege and responsibility to call forth Christ from Heaven, doesn't create God from bread and wine. He calls forth Christ and gives himself over to Him for the benefit of his people. Thus, when the priest blesses using all of his fingers, we understand that it is really Christ doing the consecration. It is Jesus who creates Himself in place of the bread and wine.
And that, my friends, is why the hand gesture is different during consecration. THAT is why the priest uses his whole hand. It's because he's allowing Christ to utilize the physical form of his body to enable Christ to pass along His Sacred Blessing.
Ah - to be a priest is to consent to a daily form of the Annunciation. Each time he says Mass, he briefly contains within himself a bit of the divinity that Mary must have acknowledged when she said "Fiat" and opened herself to the mystery of the Incarnation. Her consent to allow God to use her physical body for His greatest blessing parallels nicely with that of the priest sacrificing his physical body to allow God to manifest the mystery of the Eucharist.
And upon completion of this mystery, he (again, in persona Christi) offers this Sacrifice up to God the Father to bear forth salvation (or more rightly, bear us forth to the gift of Salvation). This is why the priest lifts the chalice and patten (or ciborium, I guess). Christ, in the person of the priest, offers Himself to His Father at the Last Supper. It's why the priest suddenly changes to first-person pronouns, too.
Our theology is so deep - so beautiful - that I could happily meditate on it for hours. EVERYTHING is meaningful. EVERYTHING is a reflection of our faith.
Never forget that.