Disclaimer: I am about to speak of something I have little to no firsthand experience of. Therefore, I may be wholly in the wrong and would be willing to accept any first-hand or empiric correction that may result from my thoughts on the following issue.
So, there's this thing that seems to happen quite a bit in movies. (Not good movies, mind you, just the kind that I watch, which may be your problem right there) The situation is usually the same: there is a man standing behind a woman, he may have a gun to her head, he may not, but for some reason or another she is totally helpless. Then, in a flash of sheer martial-arts prowess, she kicks him squarely in the face over her own shoulder, knocking the guy away, down, or out!
WOW! What a trick!!!
I accept that women can do this, because they are flexible beings. I accept that specially-trained martial arts women (spies, assassins, kindergarten teachers) could probably aim that foot accurately enough to hit the guy in the face without really seeing where his head is at that moment, because they're specially trained. However, I struggle to wrap my mind around this one concept:
How in the world does her foot, at this point positioned slightly above her collarbone, pass through her body to travel the distance necessary to hit the face of the guy who is behind her???
Now, I know, I know, generally the captor is leaned forward slightly, laughing in the woman's ear or something, and it's entirely plausible that the toe of her shoe could actually make contact with the guy's face and possible do some damage. This, however, leads me to point B:
How in the heck does that bit of foot have enough momentum to knock the larger guy backwards without impacting the balance of the woman (who is now standing on one foot) ONE BIT?!? It seems she'd have to take more impact from her own knee than Evil Guy is taking from the tip of her shoe. Seems far more likely to me that she could put out his eye with her toe than knock him back.
I mean, think of how much momentum it would take for a kick to do that amount of damage in the first place. Got it? Okay, now take away any solid base one would generally require to deliver a forceful blow of any kind and replace it with a center of gravity that is constantly shifting (part of being jostled around by Evil Guy). Now, you have got an opposing force behind you that you could use to stabilize yourself, but in this instance the source of that opposition is also the target, so it's really not much help. Finally, you have to get enough momentum to get one leg to go from standing still to over your head--not a normal maneuver for any leg, no matter how trained--in less than a second and still have enough force behind it to knock Evil Guy completely off of you (because the guy always releases the captive woman immediately following the impact; she just gets him THAT HARD!).
All right? Now, I've already confessed my inadequacy to comment on martial arts, but DANG! That doesn't seem like it should be POSSIBLE! I really wish that, just once, we'd get a profile view of that maneuver, because I'm really thinking that Evil Guy is in cahoots with Seemingly-Captured-She-Ra to make that move work. Somebody ought to look into Evil Guy's file and see if there's a history of double-agent work or other general backstabbing.
Come to think of it, I think most movie kicks are a tad dubious. In a movie, if you get kicked, you're out for a good thirty seconds. I don't know if that's really true. I'm sure a well-placed kick by a master would not only floor a man, but also knock him out. But a lot of things gotta go right for that to work, you know? In movies, generally a guy will run full-force into another guy's foot--not the heel, the hard part, and not straight-on, where the kicker would have a firm foundation to kick from, but usually from the side, or behind, so they catch the broad side of it--and the momentum swinging on one leg is enough to completely counter and overwhelm the momentum of an entire body heading full force in the opposite direction.
You know what, I obviously just need to learn how to kick people. I think that's the answer to this particular quandary, because there is just a whole lot of physics going on that I simply do not understand.