Injuries that are hard to explain to the uninitiated

Did you know that it is possible to knee yourself in the stomach?
Did you know that kneeing yourself in the stomach is excruciatingly painful?
Leaving out the long boring details, I hooked my foot on someone's hemline as he was throwing me and when it came out I managed to both kick him in the ribcage and knee myself in the stomach. At least, that's what we worked out once I regained the ability to breathe.
It could have ended much uglier. I know someone who broke her own nose doing the exact same thing.
Why do I choose to do this?