As in, take a knock to the stomach and then keel over on the ground mouthing to everyone else around you “I can’t breathe!”.
Then eventually, of course, the airways open and normal breathing takes over. But why do the lungs appear to close up for around 10-20 seconds when winded? And why do they eventually unblock?
Generally the diaphragm, the muscle that works like a bellows to move air in and out of the lungs, gets temporarily paralyzed.
Elaborating on qadgop’s answer, the area where one gets struck is called the ‘solar plexus’. It’s a cluster of nerves that radiates outwards from that area, in a vaguely sun-shaped pattern, hence the name. When that area is hit, you basically have a whole cluster of nerves struck, and your diaphragm is temporarily paralyzed.
Elaborating on Redukter’s answer, there isn’t as much muscle mass over the solar plexus as there are over other nerve clusters, so it’s very sensitive (getting poked there with one finger can drop most people, whereas getting socked full-strength on a nerve cluster with muscle over it won’t have much effect). This makes it an easy and popular target.
Elaborating on Ethilrist’s answer, therefore, you should always avoid getting punched in the solar plexus. Of course, getting kicked in the testicles is no fun, either. Hey, why does that hurt so much?
Elaborating on Kamandi’s answer, getting kicked in the testicles stimulates pain fibers that communicate with the nerves in the gut, thereby causing significant nausea, with its associated sweating and transient drop in blood pressure, accentuating the pain.
There! Now we have a nice circle elaboration! Figure eights anyone?