Subject: Why does beer overflow from a bottle when the bottle is struck sharply?

In college, many guys played the "beer overflow" trick on unsuspecting 
undergrads. It works like this: If someone has a fairly full (3/4 or more) 
bottle of beer, you can make it quickly foam up and overflow by hitting the 
top of the bottle sharply with the bottom of your beer bottle. You have to 
use a downward motion and try to contact the entire bottle "spout" all at 
once, but if you do it right, the beer will foam up and overflow in a 
matter of seconds. How does it work?  

Re: Why does beer overflow from a bottle when the bottle is struck sharply?