I still don't know the semantic purpose of this movement. Recently I also paid attention to a new Japanese women's movement that I hadn't thought of as being different from other women's movements before, until a lightbulb moment that it is different. Do you know the surprised smiling facial expression but usually not accompanied by any vocalisation, and then this open-mouthed expression with expanded cheeks is sustained while slowly looking around with an increasing level of visible amusement as if to confirm that other people took note of this "shock" reaction's occurrence? Everybody does this, not only Japanese and not only women. It is associated with femininity so when men do it, usually it's more brief and followed by another gesture, while not always because sometimes it lacks the feminine association if the context is truly something surprisingly amusing. The threshold for women to do it is much lower so they do it much more, and in an exaggerated way that I suppose probably most people think is cute. It also isn't a conscious gesture similarly to how the "sway" is not, at least it doesn't begin that way, though I'm certain the sustaining and some variants must involve a degree of volition and awareness.
Do you know what I noticed? Clear differences in the details of this movement exist between every different ethnicity! The first time I took note of this wasn't about Japanese doing it different than anybody else, it was about Chinese women doing this far more likely in such a way that involves moving their lips like a fish in slow motion and opening their eyes wider and wider in a gradual process, also even more likely than other ethnicities to put a hand near their mouth but not cover it. The Japanese variant occurred to me later and it's more subtle in its difference from other ethnicities. I believe I've seen it even in JAV too but can't think of any examples. This variation that seems more typical as a Japanese women's movement involves a bodily movement of leaning back and momentarily staring at fixed points with the openness of the mouth remaining more consistent than when other ethnicities do it. Clearly this isn't always the case, but it has enough correlation that I'm confident in pointing out this observation. I don't see enough of most ethnicities in real life to know if their variants are different, however, I am confident that Germans also do it in a unique way and that this has an even stronger correlation with language than the other variants! It's a small sample size but the reason I'm convinced is that I saw a group of Asian-German tourists (one surely Chinese-German and one surely Japanese-German but of others I was unsure) and when one of the guys said something, most of the girls simultaneously made this movement and it was a unique variant that even in the certainly ethnically Chinese and Japanese cases seemed to override what would have been the expected variants. It involved the curling of the lips and leaning forward with some erratic movement of the body! I thought back to any Germans I've ever seen and realised that it must be the variant they do.
If you can think of any JAV where this movement is made, it may help to illustrate what I'm talking about. It's possible that it would not always be the Japanese variant because I certainly have seen Japanese girls make the movement in other variants. The same is true about other ethnicities and other variants, including the Japanese variant. If I think about my own ethnicity, the most common variant doesn't appear unique but something between the Chinese and Japanese variants with possible preference for the Japanese fixation and leaning but the Chinese eye widening and movement of the hand close to the mouth without covering it. I have seen the German variant too and wonder if maybe the differences are less in the immediate first expression as this may be a truly universal and genuine reaction, but rather it begins in any unspecific way and then is somehow guided by the person's primary language into the preferred direction. If my observations about movements are statistically accurate across the entire populations, then they must relate to both the primary language (not exclusively first language or cultural factors) and cultural factors (but not exclusively language) with language making a bigger difference. However, because Americans are easy to discern from other English-speaking populations based on their movements and mainland Chinese from other Chinese-speaking populations, there must be a combination from culture and surroundings. It is interesting. When Japanese girls are born and raised here, most of them still make Japanese women's movements while even the biggest weebs still make the non-Japanese variants. Non-Japanese raised in Japan seem to prefer Japanese variants, those who lived in Japan later do not. Bigger sample sizes could help explain.