What’s up with that? When did it start? Why? Chicken or Egg, did it happen in a movie first, and then women all over started imitating it, or vice versa?
I’ve never once in my entire 42 years noticed an actual, live woman do this in person–certainly none of the ones I’ve ever kissed. Only ever on TV or in movies.
I, certainly, have never done it as a reaction to a kiss. I have, on the other hand, done it for comedic effect…
There was an old movie (which I’ve not seen but had described to me) where this affectation was seen as the sigh of Twoo Wuvv - the dame kisses Mr Wrong and the kick happens involuntarily, she kisses Mr Right and it doesn’t, so the two Misters swap roles in her mind. From the description I took it to be from the 40s or 50s but I have no more data than that. My WAG is that the movie came first.
I had a girlfriend that did this. I thought it was cute and sexy as hell. I have no idea where it started.
I dunno about the leg, but mine leans back from the lower back/hips every time I kiss her standing, kind of like that picture of the sailor kissing the woman from WWII. It’s kinda cute but occaisonally odd when she overdoes it and I’m struggling to keep balance while kissing her. Kicking, not so much, but she definitely does have a non-kissing-related movement that I’ve seen before in historical context.
It’s to kick that dog in the balls, the one that’s allus trying to hump her leg
Mine kick them pass there ears when I kiss them.
Would men prefer a knee to the groin?
Wouldn’t it have something to do with the fact that men tend to be taller then women, so a woman is “reaching up” to kiss a man, and may sometimes have to stand on her toes to do so, thereby having one leg come off the ground?
Just a WAG.
They joked about this in the first Naked Gun movie – Priscilla Presley’s leg goes up when she kisses Leslie Nielsen, then his leg goes up the same way.
I am just five feet tall and have never had this happen to me. YMMV, I suppose. 
Girl who kisses on tiptoes checking in.
I find that I subconsciously do it, probably because almost every man I’ve ever kissed was taller, sometimes significantly so (like, 10" or more). It makes for a rather amusing effect when I kiss The Boy, as he’s 5’4" to my 5’5".
However, both feet always stay solidly planted on the ground when I do this. When you’re already a little unbalanced from the combination of kiss and tiptoes, the last thing you want to do is take away one of your primary stabilizers.
You’re kissing them in the wrong place, dude.
Well if it was Scarlett Johanssons knee I wouldn’t mind, not one bit 
Quick tip: Just as the kiss is initiated, kick her in the shin!
Glad to be of service.
I was thinking this too, and I do notice I balance on one toe, while the other lifts slightly and dangles, but doesn’t bend at a 90-degree angle or more.
You know how, when you scratch a dog’s belly and find the right spot, his hind leg will kick involuntarily? Maybe the kisser has found the woman’s kicker spot.
I’ve seen it in movies going back to at least the late 1920s. I doubt it went back much earlier: hemlines didn’t go above the ankles until circa 1919 after World War I, and a kickback under a long skirt would just look awkward.
Ok, here is what happened to me. Not too long ago I met a very handsome, intelligent fabulous man with whom I had an incredible connection. We were on our way to a movie, but I had to stop and check on my animals, so he stopped in my driveway (we had met at a local restaurant).
I got out of his car and moved to the top of the driveway, near the back porch. He followed and I was glad for it.
Before I went into the house, he stood before me, leaned down and kissed me. The emotions swept around me, and I was caught in this passionate moment.
I found my foot doing something it has never done before. My right foot began to raise of its own accord and then paused, mid air, just like the women’s legs in so many films. What the heck? LOL
So, that is my movie moment. It is true and I am thankful for that moment.
Peace and light!