You're right that shaking does very little to change the equilibria. However it does a lot to change the pathway from the high-pressure equilibrium to the low-pressure one.
Let's say the can has been sitting quietly before it's opened. The CO2
is partly on top and partly in solution. When the pressure drops, there are few places for the CO2
in solution to form bubbles. It's hard for a bubble to start in the middle of the liquid, because a very tiny bubble is actually less stable than either a very large bubble or no bubble, due to the surface tension. So the CO2
slowly comes out the top surface.
If you've just shaken the can, it has many bubbles distributed through the liquid. When the pressure drops, they grow rapidly, initially expanding just because the pressure is lower and then because more CO2
flows to the gas from the liquid, just as it does at the top surface. These expanding bubbles make the fizz.
(published on 09/27/2010)