Sometimes I will try to swat a fly, but I will miss her by a few centimeters. Still, the fly might either fall down still moving her legs, or continue to fly, but much slower than before.
Is the fly somehow injured by the loud sound/intense airwave of the clapping? Or she simply gets dizzy?
you know its a she fly? nice work 
In some languages animals and things have a “default” gender. In greek, a fly is female. 
Assuming you didn’t hit the fly, it’s possible the air current just ruffled up its wings a bit.
When I read the thread title, I thought it was going to ask how to kill a housefly by actually just clapping you hands together over it. This is something I do, and it almost works: when the fly lands on a surface, put your hands about a foot and a half apart on either side of the fly, and about 2 inches above the surface it is sitting on, and clap your hands together. When the fly sees your hands coming it will insinctively fly upwards, and directly into the path of your hands. And don’t forget to wash your hands
Hhhmm. I’ve read that flies take off backwards, so my technique involves an arena of attack slightly behind and above the nasty little bastard. That’s why when I use a flyswatter, I try to come from behind.
Even better is getting a Two-For-One!
Flies possess balance organs called halteres, which are located behind their wings, and are actually modifications to the hind wings found in other insects. Perhaps the concussion affects them? I swat behind them, too - I don’t know if they physically HAVE to take off backwards, but they usually do.
I do that trick too, but always as an attempt to catch them, not squish them.
I became sort of locally famous for being able to kill flies by clapping above them. They take off into the kill zone if you get close enough. Living for a couple years in SE Georgia, there were enough flies for everyone, believe me.
A successful hand clapping kill is required to be followed by the “Got the Sucker” exclamation.
It can also be done with one hand using a long sweeping motion. The idea is to catch the little bugger in your hand, shake it like a pair of dice and slam it against the wall. Snake Eyes!!
Me, too…except I shot rubber bands at them instead of clapping above them. A little messy, but I didn’t have to chase them around.