In the Missale Romanum as promulgated by Bl. Paul VI, and the subsequent editions based thereon, the rubrics direct that one is to bow during the Creed at the words “Et incarnatus est de Spiritu Sancto ex Maria Virgine: et homo factus est.” On the Feasts of the Nativity and Annunciation, however, the rubrics direct that one is to genuflect at those words.
The General Rubrics for the 1962 Missale Romanum, by contrast, state, “ad verba autem Et incarnatus est, in symbolo, sacerdos celebrans semper genuflectit.” In the text of the Ordinary itself, one finds the rubric “Hic genuflectitur” immediately preceding the words “Et incarnatus est.” In short, the instruction in the Extraordinary Form is to kneel, rather than bow, at this portion of the Creed.
The creed, however, is one of the portions of the Mass that the priest may read at a different pace than the choir sings. Thus, the priest will genuflect by himself as he reads the text, and then may genuflect later with the congregation when the choir reaches the same line.
Here, the celebrant reaches the et incarnatus est in his own reading:
In the celebration seen here, after completing his own reading of the creed, he moves to the top of the altar steps and waits for the choir to reach the et incarnatus est; he then genuflects with the congregation as that portion of the Creed is sung:
Afterwards, he returns to the sedilia until the creed is (almost) completed. The whole sequence:
Where the priest above waits for the choir at the altar steps, he could also have returned directly to the sedilia; the general rubrics continue on from the portion quoted above to direct him to bow if seated at that point (but to get up and kneel on the Annunciation and Christmas).
Note that in the solemn Mass seen in the video, the deacon begins the preparation of the altar after the ministers return to the sedilia. This non-linear, layered quality of the liturgical action was an old feature of the Roman Rite, but was almost entirely extirpated in the 1970 Missal. It can, admittedly, be extremely disorienting at first for those used to assisting at the Ordinary Form.
 Roughly, “at the words ‘et incarnatus est’ in the creed, the celebrating priest always genuflects.” Rubrics of the Roman Breviary and Missal, 52 A.A.S. 597, 683–84 ¶ 518(b) (Sacred Congregation of Rites, July 26, 1960).
 St. John XXIII, Missale Romanun 220 (Church Music Ass’n of Am. ed.), available at http://www.sanctamissa.org/en/resources/books-1962/missale-romanum-1962.pdf.