Exactly what happens when you get the wind knocked out of you?

My email account has these little messages that pop up as I'm checking my email. Like...Exactly what happens when you get the wind knocked out of you?

I find myself intrigued by these questions so I check them.

BTW I hate it when I get the wind knocked out of me and am thankful it hasn't happened in like 15 years.

The last time I can remember was when I was skiing with Chad someplace in Maine, my ski boot broke, and I hit a tree trunk smack right on my human trunk at what seemed like 30 MPH.

Thought I was going to die at the time. Luckily I was young and recovered.