GIRM 275. A bow signifies reverence and honor shown to the persons themselves or to the signs that represent them. There are two kinds of bows: a bow of the head and a bow of the body.
- A bow of the head is made when the three Divine Persons are named together and at the names of Jesus, of the Blessed Virgin Mary, and of the Saint in whose honor Mass is being celebrated.
- A bow of the body, that is to say a profound bow, is made to the altar; during the prayers Munda cor meum (Almighty God, cleanse my heart) and In spiritu humilitatis (Lord God, we ask you to receive); in the Creed at the words Et incarnatus est (by the power of the Holy Spirit . . . made man); in the Roman Canon at the words Supplices te rogamus (Almighty God, we pray that your angel). The same kind of bow is made by the deacon when he asks for a blessing before the proclamation of the Gospel. In addition, the priest bows slightly as he speaks the words of the Lord at the consecration.
At the mentioning of the names, everyone should bow. IOW, this is not limited to the verbal responses of the individual, but whenever the Divine Persons or Saints (F,S,HS ; Jesus ; Mary ; Saint of the Day) are named by priest, deacon, or congregation.
If you watch traditional Latin Masses, this is readily apparent. That practice has continued de jure, but rarely de facto.
That having been said, one should not make a show about it. I remember when I went with two of my buddies to Europe on a pilgrimage, and we went to Notre Dame Cathedral in Paris. We went to Mass in the apse, and it was beautiful. However, everything was in French, so I didn’t understand any of it (an argument for the return of Latin?). But what I remember most about that Mass was some guy in the small congregation who, at seemingly random moments would get up from his pew, face the altar, and make a dramatic body bow, and then sit down. No one really said anything, but it was kinda silly to see. I figured out that he was taking GIRM 275 a little to literally, and was drawing undue attention to himself.
A slight head bow is appropriate, but don’t make it into a rubrical show of piety