The Fight-or-Flight Syndrome, a term coined in 1929, apparently, is part of the hardwiring, not just in humans but in other mammalian species. Updated by more research, the terms "freeze," aka "tonic immobility," and even "fainting" have been added in many places.
A few spots to learn about the thing -- here, here, or here. Be warned that the first couple links are dense with medical terminology and not easy reading. The third is a wiki, and easier to understand.
This one concerns stuttering, but has some good information. Or this one, on the dynamics of the FoF arousal cycle.
Boiled down, what it means is, that if faced with sudden surprise, especially perceived danger, humans will generally do one of several things, and more or less in this order: Freeze, run, fight, or faint.
Um. Since this is a topic that comes up frequently in discussions of martial arts and street violence, ways to break the freeze as quickly as possible are good things to know, and I refer you to Rory Miller's book/blog for more on this. Recognizing that you probably will freeze if something goes booga-booga in your face readies you for the notion that figuring out something to get your ass in gear and moving is a swell idea.
What, exactly? I dunno for sure. I've thought that screaming might do the trick, since such engages a different part of your brain and could let you take the brakes off. Lot of war cries throughout history, and even in nightmares, the scream is sometimes what wakes one up.
Blowing out your air, maybe the ability to release one's rage might work, as well.
Recently, somebody sent me a link to a video that demonstrated this tonic immobility and ways in which it might be broken. It involves a puerile, somewhat idiotic activity called "sharking."
Being an old fellow who lives in a sleepy suburb, I was not aware of such a thing, but the gist of it is this: Some brain-damaged young man, usually wearing a hoodie, with the aid of another dillwitted fellow who has a videocam, stalk attractive young women on a warm and sunny day when the women are dressed for the weather. The first guy runs up to the woman, jerks her off-the-shoulder blouse down, or her skirt up, then hauls ass, as the second loon records the molested woman's sudden partial nudity.
The spiritual descendent of the wedgie, and about as infantile.
Aside from being generally stupid, this is a sex crime pretty much everywhere, and apt to get the louts registered as perverts, jailed for assault, or their asses kicked by irate boyfriends if they get caught.
My correspondent sent me a link to a video purportedly done in Russia, although it is apparently all the rage in Japan, as well. Ah, those foreigners and their strange ways!
The reason he sent me the link was not the PG-13 rated visuals as boobs popped out or thongs were revealed, but for the women's reactions.
Nearly every one of the women suddenly beset by Zippy the Pinhead and his budding cinematographer pal froze when attacked.
The first reaction after that was generally to cover the uncovered anatomy, to turn and face the now-running-away molestor.
The women who moved the quickest were those who screamed. Usually this was wordless; a couple of times, what I assumed was a Russian curse. In one case, a woman almost immediately swung her purse at the fleeing felon, and in another instance, a woman gave chase and you could see she was ready to bash the sucker's head in if she caught him.
There are a couple of versions of this vid around. The shorter one apparently is blurry, while the longer one, much sharper in resolution, slows down the vid at the moments of contact, and you can see the women's faces clearly. (And no, I'm not giving you the link, my site is already getting enough traffic. Google it yourself ...)