Bill, that part of the Mass is called the epiclesis, or the calling down of the Holy Spirit.
Make Holy, therefor, these gifts… Send down your Spirit like the dewfall… Or similar words are used, and the priest holds his hands above the bread and wine.
Traditional Roman Catholic belief is that transubstantiation occurs a bit later, at the words of institution - ***this is my body ***and this is the chalice of my blood. Eastern Catholic Churches have, to a greater or lesser extent, held that transubstantiation occurs at the epiclesis.
The epiclesis for me is an amazing part of the Mass. So beautiful, when the Holy Spirit is present in the Church in a very special way. The Holy Spirit was present with Mary at the annuncation to make Jesus present inside of her, and the Holy Spirit is present in the Church at the epiclisis in the same way, to Make Christ present on the altar. It is extremely beautiful, and a holy moment for me. I am always in awe.
I don’t think we can really know the exact instant when transubstantiation occurs, just like we can’t know the instant in which Christ became present inside of Mary. All we can do is believe that at some point while the priest is calling down the Holy Spirit and saying the words of institution, that it becomes the Body, Blood, Soul and Divinity of Jesus Christ.