I remember when Bernard Hopkin knocked out Oscar De La Hoya. I remember folks shouting that De La Hoya took a dive. But unless you've been punched in the liver, it's hard to realize how much pain that little area can create for you.
Grantland has an excellent, but short, article on the art of the body shot, HERE. The author explores the history of the bodyshot over the last 100 years and up to the July 23rd Amir Khan vs. Zab Judah fight. He also talks about the physiological reasons that the body shot can be so effective.
The knockout is right around 4:30. Well worth watching the whole video, though.