From: G Henslee 
Newsgroups: misc.consumers.house rec.autos.tech alt.home.repair
Subject: Re: B&S Engine Off-season Storage: drain gas tank or stabilize?
Date: Fri, 22 Apr 2005 10:23:01 -0700

Mark wrote:
> I add Stabil on the last usage and run the engine till the Stabil gets
> into the carb.
> 
> If its a plastic tank (lawn tractor), I then run the tank and the carb
> dry and I figure whatever gas is left in there has Stabil for
> protection.
> 
> If it's a metal tank (motorcycle) , I close the valve and let the carb
> run dry but keep the tank full.
> 
> What does Stabil do anyway, anybody know?
> The gas must still evaporate which is why I like to let the carb run
> dry.
> 
> 
> Mark
> 

According to them:

STA-BIL is a blend of scientific additives all of which act together to 
prevent fuel from undergoing degradation and oxidation during prolonged 
storage. STA-BIL acts as a protective wrapper around fuel molecules so 
they cannot combine with oxygen or other molecules to form new "bad 
actor" molecules.

http://www.i4at.org/surv/sta-bil.htm

Google is your friend.