To pray is to talk with God. To invoke God is to summon His force and presence inside of us, a form of prayer or communication with divinity. When Christ descends down from the superior worlds in order to intersect within our heart, that is a high form of prayer, an invocation and intermingling of terrestrial with sublime forces. We perform this precisely through transmutation, the Eucharist, sexual magic, conjurations, etc.
Joyful in hope, suffering in tribulation, be thou constant in thy prayer.
Benedictis, qui venit in nomine Domini. Osanna in excelsis.
"Blessed is he who comes in the name of the Lord. Hosanna in the highest!"