Now suddenly in an address about tznius to the Yeshiva he was saying that women and girls need to be careful not to swing their arms while walking in the street.I've never heard of this, even amongst frum women. But it is not just a problem in subjectively deciding what is modest and what is not, and formalizing it for everyone. There is another issue with this -- it is not for nothing that human beings instinctually swing their arms when they walk. This serves as a counterweight for maintaining balance. (For example, when designing robots, researchers trying to mimic human locomotion have the robots swing arms to maintain balance.) Without swinging arms, people would walk more stiffly and more precariously. There are real human side-effects of making such a proclamation. I wonder if this Rav tried walking for a few days without swinging his arms before imposing this new restriction on the female Jewish population.
This is not unlike the previous tznius issue I posted about -- slits in women's skirts. There, there is a strong possibility that those banning slits in women's skirts did not realize that the slits were not just there to attract attention to the bare leg, but were actually functional -- they allow women in straight skirts to move their leg forward. If so, they may have treated slits as mere "pritzus" within fashion, such that it is forbidden in any form (e.g. even below the knee, even with thick stockings underneath). But in reality, it is necessary for a woman to walk forward comfortably. The same here -- swinging the arm forward is functional, and serve not just to attract a man's attention.
So now you have women, hobbling along in these narrow skirts up to the ankle, and they cannot swing their arms to maintain balance. Next we will hear that they cannot swing their legs -- they can move by pivoting their hips.