To bow.
Which in my opinion is an unfortunate rubric.
I can’t really put into words very well why I think everyone (who can) genuflect/kneel at the et incarnatus est would do it if that was the rubric, but I do think they would.
Bowing is a more awkward movement because it takes your eyes off the people in front of you; it messes up the social dynamic of doing things together because you don’t see everyone else doing it. Seeing others doing stuff is a major part of ritual consistency. If everyone in your field of view is doing something at Mass, you are likely to do it. But if you don’t have a field of view (as in bowing) you are likely to spend half the movement peeking up and looking around to make sure everybody else is doing it. At least I do that =p. Half the time our parish decides to bow, half the time not, it seems. To add, seeing
the priest do the action while you are doing it is an important aspect. That is why there is complete uniformity of people standing when the priest stands, sitting when he sits, etc. (if they are supposed to), because they can
see him clearly doing it.
Furthermore, there is the fact that bowing has several degrees. Even if a “profound bow” is specified, that means little to the average person. Only someone who knows what that means in the liturgical setting can be expected to do it properly. Genuflecting is pretty much, you either do it or you don’t. It is true that some people do that little half-genuflection thing because of orthopedic issues, but I wager that because the time spent making the reverence in this context is so short, they would be better off not doing the genuflection/kneeling here in the first place if it would take too long to get back up.
I think this is, ironically, an over-complication in the OF. To add to that, there are two days of the year when you do in fact kneel/genuflect/whatever the rubric says in the OF. Some people may say that makes these days “special.” I understand the spirit of that. However I think it is very poor foresight, practically speaking, to expect this to work well and consistently. In my experience some parishes do it, some parishes don’t do it even though they’ve been reminded, some parishes don’t even mention it I guess because the priest either doesn’t know or doesn’t think it would be worth the effort. I sympathize with that latter view. Random liturgical reminders inserted in the Mass are distracting imo and they are slightly heartbreaking when they interrupt an otherwise beautiful liturgy. “Please kneel today at the ‘was made flesh.’” “Wait what?”
This does not work well: “Well let’s bow 99% of the days of the year but hey let’s kneel two days a year, yes, everyone will remember that and it will turn out splendoriously.”
Genuflection or kneeling would be an easier and much more reasonable rubric imho.
And yes, before anyone raises pitchforks, lights the pyre and calls Cardinal Mueller, the OF is valid, licit, etc., and so on,
inserts-socially-mandatory-(…for-traditionalists-only…)-disclaimer-times-one-thousand, ad infinitum. . . .