Recently I was asked why I make a profound bow or "full bow of the body" during the recitation of the Credo during the Catholic Latin rite celebration of the Eucharist where the rubrics direct us "all bow during these two lines":
by the power of the Holy Spirit
he was born of the Virgin Mary, and became
while others around me either exhibit no movement whatsoever, or nod their chin 3-5 cm, or simply bow their head.
234b. Query: Some of the acts of reverence by both the celebrant and the people have fallen into disuse, for example, the profound bow to be made in place of the former genuflection at the words announcing the mystery of the incarnation in the "Credo." Are such gestures still to be observed?
Reply: Clearly people should express their faith, devotion, and reverence not only by words but also by gestures and posture. All the more care should be taken about this because the gestures now prescribed since the reform of the liturgy are fewer and simpler.
Thus the Order of Mass and the GIRM assign a few instances when gestures are to accompany the words. It is enough to recall the GIRM no. 234 to recognize these various cases: "A bow of the head is made when the three divine persons are named together and at the name of Jesus, Mary, and the saint in whose honor Mass is celebrated. A bow of the body, or profound bow, is made: toward the altar if there is no tabernacle with the blessed sacrament; during the prayers, "Almighty God, cleanse and Lord God, we ask you to receive"; within the profession of faith at the words, "by the power of the Holy Spirit;" in Eucharistic Prayer I (Roman Canon) at the words, "Almighty God, we pray."" In the case of the words in the "Credo" the rubric of the Order of Mass also reads: "All bow."
It is well to remember that at the Mass of the Christmas Vigil, the Mass at Midnight, the Mass at Dawn, and the Mass during the Day, there is genuflection at the words "And he became man" (see MR pp. 153, 155, 156, 157); the same holds for Mass on the solemnity of the Annunciation of Our Lord (see MR p. 538).
For the consecration of the bread and wine the GIRM no. 234b prescribes: "The priest bends over slightly as he says the words of the Lord at the consecration." Further the priest genuflects "after the showing of the host," and "after the showing of the chalice" (GIRM no. 233); "he genuflects in adoration" (Order of Mass, nos. 91-92, 104-105, 111-112, 120-121). As for concelebrants, they stand at the showing of the host and chalice, look at them, then bow profoundly (GIRM nos. 174c, 180c, 184c, 188c).
Likewise before communion there are gestures of reverence and faith made by both the celebrant and the people who receive communion. For the celebrant the GIRM no. 115 and the Order of Mass no. 133 have ". . .then the priest genuflects, takes the host" etc.; and for concelebrants the GIRM directs: "One by one the concelebrants come to the middle of the altar, genuflect, and reverently take the body of Christ from the altar.
Then holding the eucharistic bread in the right hand, with the left hand under it, they return to their places. The concelebrants may, however, remain in their places and take the body of Christ from the paten presented to them by the principal celebrant or by one or more of the concelebrants, or from the paten as it is passed from one to the other" (GIRM no. 197).
Note: GIRM is "General Instruction of the Roman Missal."
As for the people, when they receive the eucharist standing, they are able to make some sign of reverence (GIRM nos. 244c, 245b, 246b, 247b): Not 14 (1978) 535-536, no. 11. ( Catholic Answers )