I asked a question on Martial Arts.SE about why many boxers combine each punch they throw with a rapid exhalation (making a pronounced, sharp huffing or hissing sound). Some of the answers say that one of the reasons for the exhalation is the expectation of being counterpunched, possibly to the body, which makes it desirably to preemptively empty the lungs to reduce the effects of having your wind knocked out.
This simultaneously makes sense and confuses me.
If you've had the wind knocked out of you (as I have, more than once), you know that it isn't simply a matter of the lungs being emptied - if that were the case, you could simply take another breath and replace the missing air without any fuss. Instead, having the wind knocked out of you makes it utterly impossible to breathe in for a fairly long time - in my experience, you start to take in a little bit of air after about 30 seconds, and can breathe somewhat normally after a minute or so.
The reason having your wind knocked out is so panicky and terrifying is because your body seems to turn traitor and refuse to partake in the fresh air surrounding you, and which you so desperately need. It doesn't seem to be a matter of the lungs merely emptying; it feels like the diaphragm itself has stopped working.
What is happening here? Have the lungs merely emptied? Is the diaphragm convulsing or temporarily paralyzed? Is it something else entirely, or a combination of some/all of the above, and/or other factors? And finally, does it really make a difference whether the lungs are full when the impact occurs?